Looking at the PasswordBox Styles and Templates, the default template for the PasswordBox does not seem to have the ContentControl (in the case a ScrollViewer) respond to changes to VerticalContentAlignment changes. I have attempted to alleviate this in the style below.
I believe the default style is centered for a PasswordBox with a height of 32, not 40. You may also have to change the Padding value to get it to be further centered.
<!-- Taken from the Default style for Windows.UI.Xaml.Controls.PasswordBox -->
<Style TargetType="PasswordBox">
<Setter Property="MinWidth" Value="{ThemeResource TextControlThemeMinWidth}" />
<Setter Property="MinHeight" Value="{ThemeResource TextControlThemeMinHeight}" />
<Setter Property="Foreground" Value="{ThemeResource TextBoxForegroundThemeBrush}" />
<Setter Property="Background" Value="{ThemeResource TextBoxBackgroundThemeBrush}" />
<Setter Property="SelectionHighlightColor" Value="{ThemeResource TextSelectionHighlightColorThemeBrush}" />
<Setter Property="BorderBrush" Value="{ThemeResource TextBoxBorderThemeBrush}" />
<Setter Property="BorderThickness" Value="{ThemeResource TextControlBorderThemeThickness}" />
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}" />
<Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}" />
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Hidden" />
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Hidden" />
<Setter Property="Padding" Value="{ThemeResource TextControlThemePadding}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="PasswordBox">
<Grid>
<Grid.Resources>
<Style x:Name="RevealButtonStyle" TargetType="Button">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal" />
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BackgroundElement"
Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPointerOverBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement"
Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPointerOverBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="GlyphElement"
Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPointerOverForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BackgroundElement"
Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPressedBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement"
Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPressedBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="GlyphElement"
Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPressedForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Storyboard.TargetName="BackgroundElement"
Storyboard.TargetProperty="Opacity"
To="0"
Duration="0" />
<DoubleAnimation Storyboard.TargetName="BorderElement"
Storyboard.TargetProperty="Opacity"
To="0"
Duration="0" />
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="BorderElement"
BorderBrush="{ThemeResource TextBoxButtonBorderThemeBrush}"
BorderThickness="{TemplateBinding BorderThickness}"/>
<Border x:Name="BackgroundElement"
Background="{ThemeResource TextBoxButtonBackgroundThemeBrush}"
Margin="{TemplateBinding BorderThickness}">
<TextBlock x:Name="GlyphElement"
Foreground="{ThemeResource TextBoxButtonForegroundThemeBrush}"
VerticalAlignment="Center"
HorizontalAlignment="Center"
FontStyle="Normal"
Text=""
FontFamily="{ThemeResource SymbolThemeFontFamily}"
AutomationProperties.AccessibilityView="Raw"/>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Grid.Resources>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BackgroundElement"
Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement"
Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement"
Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter"
Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Normal">
<Storyboard>
<DoubleAnimation Storyboard.TargetName="BackgroundElement"
Storyboard.TargetProperty="Opacity"
Duration="0"
To="{ThemeResource TextControlBackgroundThemeOpacity}" />
<DoubleAnimation Storyboard.TargetName="BorderElement"
Storyboard.TargetProperty="Opacity"
Duration="0"
To="{ThemeResource TextControlBorderThemeOpacity}" />
</Storyboard>
</VisualState>
<VisualState x:Name="PointerOver">
<Storyboard>
<DoubleAnimation Storyboard.TargetName="BackgroundElement"
Storyboard.TargetProperty="Opacity"
Duration="0"
To="{ThemeResource TextControlPointerOverBackgroundThemeOpacity}" />
<DoubleAnimation Storyboard.TargetName="BorderElement"
Storyboard.TargetProperty="Opacity"
Duration="0"
To="{ThemeResource TextControlPointerOverBorderThemeOpacity}" />
</Storyboard>
</VisualState>
<VisualState x:Name="Focused" />
</VisualStateGroup>
<VisualStateGroup x:Name="ButtonStates">
<VisualState x:Name="ButtonVisible">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="RevealButton"
Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="ButtonCollapsed" />
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<Border x:Name="BackgroundElement"
Grid.Row="1"
Background="{TemplateBinding Background}"
Margin="{TemplateBinding BorderThickness}"
Grid.ColumnSpan="2"
Grid.RowSpan="1"/>
<Border x:Name="BorderElement"
Grid.Row="1"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Grid.ColumnSpan="2"
Grid.RowSpan="1"/>
<ContentPresenter x:Name="HeaderContentPresenter"
Grid.Row="0"
Foreground="{ThemeResource TextBoxForegroundHeaderThemeBrush}"
Margin="0,4,0,4"
Grid.ColumnSpan="2"
Content="{TemplateBinding Header}"
ContentTemplate="{TemplateBinding HeaderTemplate}"
FontWeight="Semilight" />
<ScrollViewer x:Name="ContentElement"
Grid.Row="1"
HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}"
HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}"
VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}"
IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}"
IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
Margin="{TemplateBinding BorderThickness}"
Padding="{TemplateBinding Padding}"
IsTabStop="False"
ZoomMode="Disabled"
AutomationProperties.AccessibilityView="Raw"/>
<ContentControl x:Name="PlaceholderTextContentPresenter"
Grid.Row="1"
Foreground="{ThemeResource TextBoxPlaceholderTextThemeBrush}"
Margin="{TemplateBinding BorderThickness}"
Padding="{TemplateBinding Padding}"
IsTabStop="False"
Grid.ColumnSpan="2"
Content="{TemplateBinding PlaceholderText}"
IsHitTestVisible="False"/>
<Button x:Name="RevealButton"
Grid.Row="1"
Style="{StaticResource RevealButtonStyle}"
BorderThickness="{TemplateBinding BorderThickness}"
IsTabStop="False"
Grid.Column="1"
Visibility="Collapsed"
FontSize="{TemplateBinding FontSize}"
VerticalAlignment="Stretch"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Note: I have not had a chance to try this yet, it is at best an educated guess. There may still need to be some tweaks made for it to work as intended.
Hope this helps and happy coding!
PasswordBox, and for me (VS2013, Windows 8, .NET Framework 4.5), the password is shown centered, both vertically and horizontally. Please show how you apply the style to the PasswordBox and also show any other properties set on the PasswordBox. - user128300<PasswordBox Password="{Binding Password}" Grid.Row="2" />- Jozef Cechovsky