Sorry to reply so late. University is keeping me tooooo busy.
I spent Christmas 2003 in Hanoi, with two other backpackers I had met on the road. If you are looking for a blastlike ambiance, it isn't the best destination -My guess would be to head to Phuket or Bangkok. However, for the experience, it was worth it.
In fact, there is only one reason why I'd suggest Hanoi for Christmas : downtown's metamorphosis into a surreal sea of young Vietnamese occupying the streets, with several wearing Christmas hats or dressed in Santa Claus. The crowd gets so dense that no car is able to circulate in the area during the evening.
However, after about 10:00 pm, it comes to an end. Quiet sleeping city.
Early evening, we went to a chic restaurant and had a huge meal (we ate for two hours!) with wine, for the amount of... $4 per person !!!!!!! After walking the streets downtown, and going to the only Christian church of the city, we had lychee alcohol in a bar where you have to take your shoes off and sit on the carpet.
Went back to the hostel at 1:00 am only, and it was closed. I thought because it was Christmas they'd change the opening hours, but no.
In conclusion : quiet but surreal, a very cultural experience !