Seems to me that since Global Warming began, things have changed a bit. In Nov '10 I was in Luang Prabang over American Thanksgiving (last Thursday) and that sun and heat was extra strong - hardly the dry season/cool temps listed under Laos!
As for Vietnam, I like late February through March, and again I disagree with what is written, because Saigon has not had excessive heat then when I've been there in March. I consider Vietnam's March weather to be similar to that of California in spring - basically beautiful (mildly warm and dry).
[ Edit: Edited on 18-Dec-2011, at 19:15 by Daawgon ]
That's some nice peice of info, surely I'll keep it handy.
Best time to travel Indochina is October to end of March. The weather then is pretty cool, 20-26C depends where you stay, sunny, no rain... It's also lovely for snorkeling, scuba diving if you are at beach.
From April to September, weather become very hot and start to rain in July, it's not good for traveling then.
Historical averages are your best guide. Any one month can be very different than that month a year ago. This is a monthly guide for BKK. Hottest month: April, wettest month: Sept. You can do the same for each country you plan on visiting. Good luck with your planning.