Simply cannot be done.
The VPNService does not allow you to create a profile with username and password and there is no support for creating a L2TP or PPTP programmatically. Lots of people have been looking for this but Google are just not sharing their toys.
Even attempting to use the Hidden API has no support. Many VPN apps out there default to use OpenVPN-ICS or IKEV2(strongSwan) or require you to manually create the L2TP/ PPTP VPN connection on the device yourself.
Many people have requested this from Google but they either get a message saying the request is now obsolete or it is not financially viable.