Time: November to March. Dec and Jan can actually be a little cold up north, had 4 degrees at night in Phongsali around 15th of january 2015. April and May usually very hot and with risk of heavy showers.
Rental shops in CM: Pops, C&P, Tonys big bikes, Mr Mecanic
Bikes: I belive there arent many XR left, but plenty of Crf's and maybe some Klx's too, they will do the job just fine.
I dont think you will get into Vietnam with thaiplated bikes. But Laos and Cambo should be fine as long as you get papers from the rental shops that show that youre allowed to take the bikes out of Thailand.
The border (theres only one bordercrossing) between Laos and Cambo is known as a difficult one, especially in the direction out of Laos and into Cambo. The other way it seems a bit easier. So it might be an idea to ride Cambo first and then Laos. On the other hand you might only get an export permition of 30 days from Thailand so if youre planning to spend more than 30 days in those two countries it would be conveniant to pop into Thailand again after Laos and then into Cambo from for example Surin province of Thailand. And by that get a new 30 days export permition.
Routes: In Thailand there are so many roads up north its hard to recommend one instead of another. Do the Mae Hong Son loop of course, then much of Chiang Rai, Phayao, Nan, Tak, Chiang Mai provinses are great for riding. Uttaradit and Loei are also great.
Im sure there are guys with better knowledge than me that can give you more detailed advises.
Laos - if you begin in the north and cross over from Chiang Khong to Huay Xai, then Luang Namtha, Oudomxai, Nong Khiaw, Phonsavan, Luang Prabang, Vang Vieng, Vientiane, and then south via Thakek, Savannaket, Pakse. Just a suggestion, other routes and detours possible.
