[BirdOPrey5]

I've never actually seen multiple user ranks being displayed and couldn't find the option for it when I quickly looked, but assuming this is something built into vbulletin and not a mod search the php files for: "$post[rank]"
In one of those files should be a line where it builds the ranks, look for "<br>" or "<br />" and replace it with just a comma and a space... ", "

I'm just thinking out loud here, never tried or cared about this myself.