It is illegal to ride a bike over 50cc's without a valid Vietnam Motorbike License.
The laws have changed, I believe because of the amount of motorcycle accidents by Foreigners over recent years. You, until recently, were able to go through a day course with the help of a training school, pay 500,000 VND, and get a bike license, but this has been suspended, I guess while VN transport people take a look at the laws, and toughen them up for foreigners to get a license.
Those that already have a VN bike license are of course fine, and I think you can change your countries bike license over subject to some visa requirements.
For those who don't have a bike license, it seems there will be alot more involved, and a longer time involved, and certainly a more expensive process than it was before.
Of course, the reason why having a license is important due to insurance cover in case of an accident. Recently, a foreigner hired a bike, had no license, and ran into a cyclist, who died of her injuries. What is worse, he was p1ssed... He got 3 years Jail, and his family had to pay 200 million VND as compensation.
So, I'm thinking about a few things here:
1- Is there actually a Vietnamese Insurance company that will cover you even if you are legal?
2- Will 50cc bikes become more desirable, considering you don't require more than a drivers license?
3- I guess it comes down to the same old rule, have a donation ready for the Police in case you get pulled over, keep a wad of cash in case you fall over, and need medical costs, and what ever you do, don't hit anything, or anyone, ever.... especially if you are drunk!