Your css file should be in publicly approachable directory.
Application directory (application or APPPATH) is not publicly reachable.
So your CSS file should be somewhere next to index.php file of your application.
It is common to have assets/css path started in your website root:
-application
-system
-assets
--css
---vehicle.css
-index.php
-.htaccess
And if you make your web application code that uses CSS files dinamicaly regarding of route used, you can resolve it by adding more related subfolders:
-application
-system
-assets
--css
---vehicle
----style.css
-index.php
-.htaccess
This way, after url helper is loaded, you could reach css link with:
<?php echo base_url('assets/css/vehicle/style.css');?>
Eventualy you should include assets location in .htaccess file enabling Apache server to allow reading of it.