if (isset($bbuserid))
{ do stuff }

Instead of "do stuff" you'll want to use a MySQL query to grab the username and password from the table "user." based on the $bbuserid variable.

I then simply used something like this:

<b>Username: </b><input type="text" name="user" value="<?php echo("$theusername");?>"><br>

If the variable is empty, then it isn't filled in...if it is - then its filled in.

It's all part of this system I've got integrated with vBulletin - when it's all done and officially launched I'll post it here for all to see...it's quite cool.