That way (vietnamese bike into Laos and back) might just work out, Ive seen some people with Minsks in Laos &Cambodia who´ve done just that. Or they hadnt actually gone back to Vietnam at that stage, but at least they´d managed to get those bikes out of the country.
Getting any foreign registered bike into Vietnam (or China) is a different story altogether, and very unlikely to happen without significant pre-planning, and literally miles and miles of red tape. If someone, for example, tells you its possible to go Vietnam with under 175cc bike, dont take it for granted. There are even some travel stories with topic "vietnam-with-my-own-bike" that are apparently just lies made up, thats how hard it is to succeed.
So if you have that chance, I would buy/rent in Vietnam, then go to the other countries, and not the other way round, because its just not going to work.
There are boats transporting people on the Mekong, but maybe someone else can tell you if its possible to cross from China into Laos aboard them or if you have to cross somewhere else. The boats go some 100-200kms into China, and in Laos they go to Luang Prabang, where its sometimes possible to find a boat that´ll take you to Vientiane. Depends on the water height I guess.
You might want to check out The Golden Triangle Rider Thailand/Asia Motorcycle Touring Information
forums, there´s a lot of info about the area.