Sorry I´m unable to answer all questions, but I´ll at least try a few!

Burma: might be possible to make a short distance from the northernmost Thai border. But to ride all the way to India - better just forget it.

You´re right that VN plated bikes seem to be able to cross into other countries, and return to Vietnam, and it´s known to be very tough to get any other bikes there. The problem will probably be the selection of newer and bigger bikes in VN, not so good especially if you plan to ride 2-up (Thailand or Cambodia probably have better options for that). But of course smaller and older bikes might be just fine for your trip, it really depends on how you like to travel (and how much of a mechanic you are!!)

Money: most of SE Asia used to be dirt cheap, if you came from the Euro zone, but because I havent been there for a couple of years, I don´t know how Euro´s crisis might have affected things, that´s why I don´t dare to guess a budget. But it´ll still be a lot cheaper than Europe I think!

Weather in China in March - you´ve got to be kidding, the freaking country´s almost the same size as the US, and it´ll probably have almost every imaginable kind of weather somewhere at any given time. So really depends on where you are! Areas close to VN & Laos should probably be among the warmest. SE Asia June to October is mostly the rainy season, not the very best time for riding bikes (if you compare how lovely it is over there around Christmas!) and especially some unpaved side roads could get interesting sometimes. But I´m not saying you could not do a nice trip around that time.