Hmm think I may take you to Temple Club and then walk you through the uber-cool Mr. Au's Furniture Hellhole this time Andy
OP - As RJMan said, Dung on Le Thanh Ton Street is the best tailor I have found in Vietnam....and lord knows I have been through a few.
Dung's store is in Saigon, on the street at the back of Ben Thanh Market, half way between the market and the New World Hotel. Nice store, and his wife runs the cloth import store next door, where they have a pretty ok selection of cloth. Better than I have seen elsewhere in VN.
Hoi An is regarded as the tailoring epicenter of the country, but to be honest, all I find there are backpackers in ill fitting crap. I made a tux, a double breasted suit and a peak lapel single breast when I was up there a year or so ago. I now have three more unworn suits. Dirt cheap, and they do bang em out fast (well within 24 hours)...but I guess that should tell you something on its own.