[welo]

Okay, I figured it out. After another hour or two of useless code playing and searching to figure out exactly how the stylesheet was sourced, it finally ocurred to me to look in the database. Duh.

Lo and behold, a css field resides in the prefix_style table with the entire <link rel= style statement, which will look something like this:

<link rel="stylesheet" type="text/css" href="clientscript/vbulletin_css/style-bcb1922cb-00104.css" />

All you have to do is go into phpMyAdmin and prepend the existing href= with the http path to your site. F.ex.:

<link rel="stylesheet" type="text/css" href="http://my-forum-site.com/clientscript/vbulletin_css/style-bcb1922cb-00104.css" />

Note this will also create an absolute path for the url to the stylesheet for the whole board. Like John pointed out however, soon as you change your CSS in the ACP this will be written over and you have to go back in and prepend the url again. From what I can see however, it doesn't mean you lose your CSS for already sent emails. Even if the CSS is changed the clientscript/vbulletin_css directory will still contain the old files (unless you go in and delete them).