Hi aahmadi83

Yes, to enter/exit Vietnam indipendently with your vehicle (motorcycle, car, motohome...) as they required a permit from the Directorate for Roads of Vietnam (Ministry of Transport) then you can get a temporary import/export from the border for your vehicle.

To get this permit, you will need to contact a local tour operator from Vietnam who can do this job for an extra fee.

For your long trip you can ship your motohome from USA to Vietnam. It may take up to 6 weeks but shipping is not expensive. Why you should bring your own motohome as it is under your name (owner name). You also can get the Carnet de Passage (they don't issue in SE Asia (accept Malaysia) as most of the countries will ask fro the Carnet de Passage.

To buy a RV in SE Asia for this trip doesn't seam to work or you may have to get a 3-6 months visa for Malaysia and get a resident contract...and hire some garage to organize this for you for an extra fee (if you prefer to buy, I strongly recommend to buy in Malaysia or Thailand) as they are only countries can easily accept for foreign owning a vehicle under their name. But buying an used vehicle in SE Asia you must know the history of the vehicle (if it was floaded...) as they could give you lot of trouble along the road.

Your trip sound very interesting and since my experienced it is no problem for the road from SE Asia (Vietnam, Laos, Cambodia, Malaysia, Thailand, Myanmar, India, Bhutan, Nepal, Iran...I would drive over Iran, Turkey, Bungary, Greece, Albani...I prefer Easten Europe than the West as it is cheaper, more culture, nature....

Good Luck!!!

P/s: Here is a link you can contact one local tour operator in Vietnam for information about overland crossing borders permit MOTOTOURS ASIA: Overland crossing borders motorbike tour Southeast Asia
They do motorcycle tours around SE Asia so hope they give you advices