Anniversary dinner in downtown Orlando?
Rice Paper is fab--I eat there all the time--but it's not downtown. My favorite Vietnamese downtown is Anh Hong but some posters don't like it. I've heard only good things about Lac Viet. For posh downtown, you might try The Boheme. Citrus isn't posh but it's stylish and good. Or, head to Thornton Park for trendy Hue, Beacon or Graze.
I'd recommend the new restaurant Ceviche, on Church Street downtown. I haven't been there yet, but it has been getting rave reviews on this board. The construction makes getting down there a bit hairy, but it looks like an awesome place, with a creative menu of tapas to satisfy all tastes and moods. Here's their website: http://www.ceviche.com/directions.php
For dessert, The Dessert Lady recently opened right near Ceviche, and you can't do much better than her cakes and pies and classy surroundings: http://www.dessertlady.com/
Most of the Vietnamese restaurants are clustered together on Colonial Drive (SR-50) and Mills Avenue (SR 17-92), east of downtown. You can't miss the area! There are conflicting opinions on this board as to which is best, but I strongly recommend Lac Viet, which is a little further east than the concentration of Vietnamese places, in a stand-alone, yellow building. Their food is phenomenal, prices are reasonable, and their brand-new sushi bar is surprisingly awesome as well. I also like the more casual Little Saigon, but that one gets more mixed reviews around here.
Sadly, Orlando doesn't have a "Chinatown" per se, and I have yet to find a really outstanding Chinese restaurant here in town. I can tell you that the China Jade buffet near the Fashion Square Mall on Colonial Drive is pretty mediocre, though.