Correction on Vietnamese Registration
Sorry, a bit off topic.. however, a foreigner certainly can register a motorcycle in Vietnam, in fact it is reasonably easy if you go to the right place and can pay the steep 'fees'/taxes. I am riding a legally imported, legally registered BMW G650GS.
There are a number of odd bits of rumour circulating around the internet on motorcycles and Vietnam:
1. Can't get a motorcycle over 175 cc. Not true, but they are rare and expensive. You can buy one, but renting one is limited to a few shops in Hanoi (organized tours).
2. International licenses are not recognized. True sort of. But in practice - I've not heard of anyone riding on a foreign license that faced anything worse than a 'fine'.
3. Foreigners cannot get licenses, import bikes, register bikes, own bikes, ride bikes... bla bla bla. All possible, if your are willing to take time, pay money and have a vietnamese speaking friend.
However, I agree with all of the above comments that are above. If you want to come to Vietnam and ride... make your life easy, fly here and rent, or contact a tour operator who can help you bring in your bike (read pay $1000 USD or more).