The licence question catches almost everyone. An International Driving Permit issued under the 1949 convention is not accepted in Vietnam, and that is the one most countries issue.
Converting to a Vietnamese licence is possible, but it needs a residence card valid for three months or more, which a tourist visa does not give you.
The part people miss is the insurance. Riding without a valid licence is usually enough for a travel insurer to decline a claim, and a serious accident here costs far more than the bike ever will.