I'm just back from doing that. Legally (and in most cases for travel insurance purposes) you require a Vietnamese motorbike license, which can be time consuming to get -- we were told at least ten days, and you require a motorbike license in your home country to be able to get one.
We decided to ride without one.
Hire companies are all over the tourist district of Saigon and will hire without a license but will require either your passport or a hefty deposit (US$700-800) on the bike.
If you are pulled over by the police, we were told to pay 100,000-300,000 VND on the spot to avoid more problems. In practise we were only pulled over once and that wasn't regarding a license (I was speeding and the officer just asked me to slow down).
Also, not all rental places will hire you a bike to take it into the Delta (where we went) the first three shops turned up down.
Just to note, if you ride without a valid VN license and have an accident, your travel insurance most likely will not cover you.
THANKYOU FOR YOUR ADVISE. IM ONLY IN SAIGON FOR THREE DAY'S BEFORE HEADING OFF TO SIEM REAP... I REALLY ENJOY GETTING ABOUT SIGHTSEEING AND THERE IS NO BETTER WAY THAN JUMPING ON A SCOOTER AND DISCOVERING PLACES. I'VE RIDEN SCOOTER'S IN THAILAND AND BEEN OK WITH THE MANIC TRAFFIC. HOW WOULD YOU DESCRIBE SAIGON TO A FIRST TIMER SOMTAM 2000?
Like a small version of Bangkok, without the mass transit and with a bucket load more motorbikes! Scooting around town shouldn't be a problem but if you're never seen it before the traffic can be a bit mental.