This is a bit late (a year), but, for future reference.
For Vietnamese try Saigon Pearl, it get's decent reviews and some of my friends have said it's very good (for what thats worth).

While Williamsburg appears to be the dredges when it comes to eating out, there are some very good local places (almost all off of Richmond Road, which we avoid - especially during turon season).
On Richmond Road, or close to it & the Colonial area are: Fat Canary, Blue Talon Bistro, Barretts, Aroma's (more a coffehouse), Green Leafe Cafe (excellent tap selection - plenty of microbrews - my favorite place in the area for drinks), Food for Thought, 5 Guys.
Somewhat off the path - we love Jimmy's Oven & Grill - pretty far down Richmond Road, or, their new location in Monticello Marketplace (Target Plaza). New Town (a fairly new development with retail & housing) has a pretty good selection of restaurants, most of which are local or local "chains" - Buo Amici is a good Italian restaurant there, Iciban has great asian, another Green Leafe Cafe (spotty service in the past, but, a nice outdoor area, again, good for beer).
That's enuf for now - the point is - it's not as bad as it sounds

-chris