Just got back from Myanmar, Cambodia and Vietnam. I was on a private tour.
I went to Siem Reap for Angkor Wat and then picked up a Mekong River cruise from Phnom Penh to Ho Chi MInh City. I don't feel I need to go back to Cambodia. Angkor Wat was definitely worth visiting and I recommend getting a guide there to get the most out of the various places.
Getting to and from Siem Reap can be a bit of a challenge. We flew there from Bangkok on Bangkok Airways (an airline which I recommend highly) and from there to Phnom Penh on Cambodia Angkor Air - there's also a reasonable bus service now between the two which takes 6 or so hours - it's a new one with better buses. Or you can find private car service (ask your guide to help with this). Travel within Cambodia seems to be a bit of a challenge. We applied for visas on line before we went (evisa) and I recommend this as it saved us a long line up upon entry to Cambodia.
I think Vietnam would be a good place for you to start - we barely touched the country and it seems to be booming, at least in Ho Chi Minh City. You need a visa before you leave. We applied for our visas in advance. You do not need to send in your passport - just an application form which you can get online, and some photos. Our visas were mailed back to us and we paper clipped them into our passports.
The weather in Ho Chi Minh City was about 34 degrees Celsius last week. It is dry season and we didn't have any rain. I think this is a good time to travel. We were told that it rains every day in HCMC in rainy season. Other parts of the country may differ so you need to check this out in advance.
I was bothered quite a bit by mosquitoes, although my travel companion didn't have the same problem.
If you do go to Vietnam, be aware that they celebrate New Year at the same time as the Chinese do and it's a major holiday. Many of the shops and restaurants were closed last week - many for at least a week, so this made it a bit difficult for us as visitors.