[kh99]

Well, to check if a username, password combination is valid in the vbulletin database, you'd do something like:

if ($entered_password == md5(md5($password) . $salt))
{
   // valid password
}

where $entered_password is the password that was entered, and $password and $salt are the fields from the vbulletin user table where the 'username' column equals the entered username. (I hope that makes sense).

I don't know how to write the code for you because I don't know the context of where you're trying to put it. But if I were doing it I'd probably do a couple 'SELECTS" to get the information I needed from the database, then write php to check (as opposed to doing it all in one sql statement, although that may be possible).

Also I wanted to mention that if you do write something like you posted above, don't use fields from $_POST[] directly in a query string because you don't know what they might contain. At the least, you should use mysql_real_escape_string() (or the equivalent if you're not using the mysql_* functions) to make sure any special characters are escaped.