So did you change all the same stuff in vBulletin itself too?
thread table has postuserid, along with postusername, and lastposter is the text name of the user as well.
forum table stores the whole title of the last posted thread, along with the threadid, and also keeps up with the lastposter userid and username.
All this is done for a reason... Lots of joins will kill you in a high traffic situation. (But, you already know that since you're a graduate of database design.
)