You will be there are them busiest and most expensive time of the year. I recommend that you not be at a beach the week between Xmas and NY.

Have you been to Angkor? If not, it's well worth the trip. It is truly one of the wonders of the world. Get a copy of Dawn Rooney's book, Angkor: A Guide to Cambodia's Wondrous Temples to study up and see how much time you want there. It takes three full days just to see the so-called major temples. If you love temples, you may want a week (or more) there, if temples are only marginally interesting to you, you might want to skip it, as it is costly to fly to Siem Reap, and costly in terms of time and patience to go overland.

I think Laos is well worth your time as well. But you have just a month and there are many months worth of things to do in these three countries.