My husband and I are planning to go in January for 2 weeks. We would love to have thoughts about the following itinerary:

Arrive Hong Kong
3 nights Hanoi
2 Nights Hoi An
2 Nights Saigon
3 Nights Siem Reap
Overnight Hong Kong to leave for NY.

We did however, want to go to Thailand. We didn't know how to fit that in unless we just go to Bangkok and would probably have to cut a night like, 1 of the nights in Saigon.

Any thoughts and comments would be greatly appreciated.

2 nights somewhere is just one day. Frankly, you already have too many destinations. Don't add more. Decide what is most important to you and choose those places. Any chance you can add a couple of days/

100% with Kathie. In only 2 weeks you've only really got time to do justice to one country.

If you're going to Siem Reap mainly for the Angkor Historical Site, then be aware than January is Peak Season, and the place will be absolutely mobbed. Not much fun really. We were there there last March, and tbh Angkor Wat was disappointing, although the rest of what we saw in the vast Historical Park was great. Do it by tuk tuk from your hotel, great fun.

I've not been to Vietnam, but in Cambodia as well as Siem Reap you should also consider Phnom Penh with a side trip to Kampot and Kep, and Battambang.

Skip Hong Kong and use the time in Vietnam, which deserves every minute. From New York you have lots of choices, flying both east and west. Delta,for instance, will take you to Narita where you switch to Vietnam Airlines. Go into Hanoi and out of Ho Chi Minh, or vice versa. Vietnam Airlines is an effecient outfit, flying relatively new equipment with friendly cabin service. Other Asian airines make connections through other Pacific cities. But you can also connect in Europe or even the United Emirates to get to Vietnam.
You don't have time for Thailand. Save it for another trip. Maybe the politics will cool down.

Vietnam and Cambodia are both have a huge amount to offer but with the time you have available I would stick to one. Which one depends upon YOUR interests and preferences. For me it would be. Vietnam.

If you do are intent on visiting the two countries I would spend say five days in Hanoi before heading off to Cambodia with a couple of days in Phnom Penh ( a very underrated destination) with the remainder in Siem Reap. However, I totally agree with Lancasterlad's comment re Angkor being horrendously busy. The only way to counter this is to do the opposite of what everybody else does and not visit the major temples in the early morning or late afternoon- fewer crowds but much hotter!

Thanks so much for all of your input. I will now revisit the itinerary and tweak it as per all your recommendations.