You are not recommended to drive without a valid license – this needs to be either a Vietnam’s driving license or a temporary license converted or your international driving permit. The penalties for not possessing a license when you drive can be severe: from 3 years up to 10 years if causing accident , and even 20 years if accidents causing death in addition of large fees for cover damages which will be certainly asked.

If you have car or motorbike driver license you need only to convert into Vietnamese driver license without need to get any test.
You will get a Vietnamese motorbike driving license or Vietnamese car driver license or both according to categories on driver licenser from your country.
The new Vietnamese motorbike license follows the validity of you visa/TRC or your original driver license (if it expires before your visa).
You can renew many times as long you have a valid visa/TRC and Driver License.

For that, you need convert your national license into Vietnamese one, we are able to help you about overall procedure.