Welcome to Gt-Rider.
I wish you luck in your endeavor as buying a BMW is not without its problems within Thailand,
let alone getting parts and keeping it serviced & running in Vietnam.
See the warning thread on Barcelona within the BMW section here.
https://www.gt-rider.com/thailand-motorcycle-forum/showthread.php/35702-Beware-Barcelona-Chiang-Mai!
Maybe a Kawasaki Versys may be a better bet.
However I recall a number of issues some years ago that in effect restricted the owning of bikes to the political elite and well connected.
https://www.gt-rider.com/thailand-motorcycle-forum/showthread.php/26979-Vietnam-High-displacement-bikes-get-90-tax
Apart from the issue of high import tax, there was in 2006
"a bizarre ruling restricting sales to those 30-40 years old."
The link within the thread has since been broken, and hopefully such idiotic rulings like this, have since been changed.
But I would advise some serious research
within Vietnam about what the rules regarding the ownership of big bikes in Vietnam. Also, whether you can go cross-border and ride them into Laos/Cambo, before you come to BKK to buy a bike.
You might be better-off finding an motor vehicle import agent in HCMC to advise you what is possible and what is not.
For example plating bikes in Thailand is an absolute headache unless it's through a dealer who has the particular imported or locally produced bike model properly passed through homologation. This is expensive, time-consuming and not always guaranteed on an individual basis.
Vietnam may have similar regulatory obstacles to overcome. I understood that there was a cc cap on bikes sold in Vietnam, but this may have changed for the masses as well.
Good luck and let us know how you get on.
Cheers,
Rhodie