I'll third both Baan Khanitha and Rang Mahal @ Rembrandt, both are very good indeed. Baan K's food is a little mild for me these days, but maybe I've just been here too long.

Japanese food in Bangkok is generally very good thanks to the large expat population, just steer clear from the raw fish in the cheaper places. In terms of value for money Shintake, 87/6 Soi Lang Suan, is excellent with complete meals for less than 200B. If you have a few thousand baht to throw around Bangkok is your oyster, eg. Nippon-Tei in the basement of the President Plaza (next to the Intercontinental) isn't bad.

For Vietnamese, my favorite is the small Pho chain, which serves decent grub at very good prices. One reasonably convenient outlet is on the 2nd floor of Alma House next to Central Chidlom.

And for a spot of romance, you can't miss Bed Supperclub! It ain't cheap (figure on B1500+ per head) and the food is only slightly Asian, but it's worth it. Reservations obligatory.