I have a 100% legally registered motorcycle and drivers license in China and I want to ride into Vietnam on a motorcycle. Is this possible with just those two, plus my passport and visa, American drivers license and an international driver's permit?
Motorbikes up to 50 cc can be ridden without a license in Vietnam, but this size of motorcycle is not suitable for extended travel throughout the country.
International Driver Permits from China aren’t valid as they haven’t signed the Vienna Convention On Road Traffic.
The Wikipedia article states:
The United States and China are the most notable examples of non-signatory countries
But you must hold a motorcycle license at home and have it registered on the IDP. Australia, UK, USA, and Canada have signed only the 1949 convention on IDPs, so they are not valid and you will be riding illegally.
A Vietnamese license is otherwise required.