Not sure what Bun stood for in the Vietnamese food world, I tried to find out and the consensus seems to be that Bun is an essential ingredient in a Cool Vietnamese Noodle Salad for Warm Days as described in a very detailed recipe by Mai Pham (via Fine Cooking).
Bun is a vermicelli like rice noodle.
I will see if they appear on the tasting menu tomorrow at BUN Restaurant in Soho, New York when I pop in for the opening party.
By the way, can anyone tell me what the correct pronunciation is for BUN.
We'll see if I can approximate it.