<< it's an asian thing, not just vietnamese. >>
You sure? I've never seen a Korean, Chinese, Japanese, or Thai place with the number thing, but in Fairfax, VA, where you can't take two steps without tripping over a Vietnamese place, half of them do this number deal.
I'm very unsatisfied with the "Beef Noodle Soup plus a random lucky number" answer, but if that's it, that's it.