I have a vBulletin skin on my old forums: http://mercx.exofire.net/old_forums , and I even manually converted a few free vB themes to MyBB (with all of the vB features, popup menus, etc.). Essentially, I turned MyBB into vB, but I can't give you any of my work do to legal reasons (even though I didn't actually copy any code from vB), sorry.
I didn't mean like that, I meant that I wrote some JavaScripts for popup menus and AJAX, and added a few simple functions like searches. I didn't use vB plugins, I looked at the souce of a free vB skin and painstakingly wrote a MyBB theme exactly the same. I even copied this one at one point (though I lost the XML file): http://utforums.epicgames.com/forumdisplay.php?f=249
First you will have to ask the owner then you must convert it over to mybb so that it will function right or else it won't work and you cannot up load it.
The templates are comepletely different and you will have to edit your lang PHP files. PM me and maybe we can work something out. You shouldn't have to edit the CSS, just change MyBB to use the theme's css file rather than the one in the database.