Bangkok can take as long as you have. My first trip to SE Asia was 9 full days in Bangkok - no where else. That was in the mid-1980s, and I was hooked. I've been back more than two dozen times since and still find plenty to do each time I go there. If you are really interested in the temples of Angkor, you will want more than three days. Read Dawn Rooney's book - it should give you a sense of how long you will want there. If you can do 5-5-5 (nights in each, so 4 full days in each place) that would be better.

Re the weather in Hanoi. We have just spent a month in the north including nearly 10 days in Hanoi. Temps were around 16/17 C so positively balmy by comparison with where you are now!. Not especially cold a LOT colder than the rest of SE Asia and a little bit damp and humid with very grey skies. That said, it is a city where the weather really doesn't matter that much.

Thanks crellston. I think I can handle that...we're not going for sunny beaches.

Kathie - I will get the book. Yes, I think more time there may be good. Especially considering you'll lose a 1/2 day for arrival and departure.

Thank you again all! I am getting ready to book flights (award miles) and I think I will have the following:

At least 5 full days in Bangkok. We would arrive at 11 PM and just go to bed.
Arrive 12/22 (very late) - Depart 12/28. Gives us 5 full days on the ground.

Fly to Siem Reap. Spend at least 3 full days:
Arrive 12/28 and depart 1/1. Gives us 3.5 days on the ground.

Fly to Hanoi. Spend 4 days.
Arrive 1/1 and depart 1/4 (flight departs at 9 PM). Gives us 3.5 days on the ground.

Through the magic of award miles I can build in a free stopover. So we will stopover in Beijing. Mom casually mentioned the Wall of China. So I'll make it happen. We'll spend 2 (very tired) days there and then head home.

All told we will have 13 days on the ground between Bangkok and Hanoi and then 2 days in Beijing on the way home. The flights between the cities shouldn't take up more than a half day (we will book direct flights) including arrival time at airport and the flight.

I could possibly push the flight home out a day to capitalize on an extra day in Hanoi and have it be almost the 5-3-5 that everyone here suggested.