Christmas in Hanoi, Vietnam
While in general I recommend the street food in Hanoi (it's amazing) one restaurant that was pretty good and has a nice ambiance is Bobby Chinn. It's not the most authentic Vietnamese food (compared to the street), but it's the closest that I found and has a bit of an upscale twist, i.e. using filet mignon beef.
Ly Club is a french inspired vietnamese restaurant that's not bad and has a pretty dining room (feels very special).
Some pretty restaurants with so-so food are Madame Hien (beautiful patio area) and Don's a chef bistro (pretty lake view)
I'm not sure what the holiday schedules are like for any of those, but I'm sure one of them is bound to be available for you.