The best time that I have spent in those countries has been June/July. The tourists are fewer and the weather is still lovely - with a nice reprieve in the afternoon of a light shower. The rainy season also means greener rice paddies.
The Rainy season:
From June till October 27-35.C (80-95.f)
The Dry season (cool):
From November till February 17-27.C (80-95.F)
The Dry season (Hot) :
From March till May 29-38.C (84-100.F) about 6 years ago
I was going to say the winter, but I think the first respondent has a point about going in the summer. I will say, though, that I've been to both countries twice in the winter, and the weather was really good then, although a bit chilly in northern Vietnam (Hanoi and north of that). As far as Cambodia is concerned, if the roads are still in the condition that they were in when I was there, and if you're going by bus, you may have some problems getting around in the rainy season.
You really can't go wrong in the winter. It may be the most popular time, but these are generally not countries where you would feel the presence of a lot of tourists. about 6 years ago