1. International driving license is not eligible in Vietnam and almost you can't get one.
2. Nomally, local police do not stop forgeiner except some case in Mui Ne. However, if you obey Vietnam traffic law (speed), no police has competence for stopping you.
3. For motorcyle ranging of 100, 110, 125, 150cc in Vietnam is cheap, you can get one with under 500$. But for bigger cylinder motorcyle (over 175cc), Vietnamese government limit importation, so their price are very expensive so most of them is illegal (without registration or fake registration). You can buy old 250cc dirtbike with $2500 and over.
Do you have any detail question more?