How can I force styles by Usergroup

[kh99]

You could try a plugin using hook style_fetch and code like this:

if (is_member_of($vbulletin->userinfo, 1, 2, 3))
{
    $styleid = ID1;
}
else if (is_member_of($vbulletin->userinfo, 4, 5))
{
    $styleid = ID2;
}
// etc

and of course replace ID1 and ID2 with actual style ids.