[PinkMilk]

Originally Posted by Zachery

Uh, that is php code, not css code. I'm nearly positive vBulletin doesn't use an @import in the actual css rules. It uses a standard <link> tag, in the head.

Under the vbulletin options > Style & Language Settings there is an option:
"Store CSS Stylesheets as Files?", if you check yes instead of the full css loading in page (see this sites source for example) you get something like this:

<style type="text/css" id="vbulletin_css">
/**
* vBulletin 3.8.5 CSS
* Style: 'foobar'; Style ID: 5
*/
@import url("clientscript/vbulletin_css/style-436ba35e-0005.css");
</style>

as you can see it uses @import, this is not the best option it is better to use <link> as explained in op.

The first php code snippet is what helps create this , each time you update your style a new css file is created and the function in op deletes the older stored css file.