Cambodian and Lao visa-on-arrivals are available at almost all the border crossings. Vietnamese visas must be obtained ahead of time. However we recommend getting your visa before arriving at the checkpoints. This way there is one less thing for the authorities to hold over your head in case they decide to be difficult.
Another tip we got told is to park your bike ‘out of sight’ when you check out of one country, and hide it when you check into the next country. Then once your passport is stamped, jump on the motorbike and ride like hell before the authorities can stop you. Again, we don’t recommend this (but it may work).
Vietnamese-registered motorbikes are the easiest for moving between countries, and we recommend buying one if you can. Make sure it comes with the blue registration form and keep it on you at all times.
Another thing to carry with you is an International Driving Permit, although this means nothing in Vietnam. You may be asked for this at a border crossing though, so it doesn’t hurt to have one on you.
When you are dealing with the customs officials, do not mention your motorbike at all, unless they ask. They might just be turning a blind eye to it, but by you bringing it up may make them consider asking for money for it.