Loss of the bike?
I have never heard of such a thing in thing in Southeast Asia, nor would it be a good idea. Vietnam has a sketchily enforced limit on the displacement of motorcycles, around 150cc, so any bikes you would buy or rent are either little Hondas like the Dream and the Wave, or Chinese-made imitations. They can be had for $400 or less.
It's a good rule to never insure yourself against a loss you can afford to bear yourself, and if you cannot live with a $400 loss, you probably should not be traveling.
Insurance yourself against injury? Traveler's insurance will cover that, but check with your carrier if you will need a "high-risk activity" rider (a "rider" in insurance terms is coverage for particular risk that must be specially purchased -- nothing to do with your being a rider of a motorcycle). If you are going to ride, definitely definitely definitely get evacuation insurance: if you are in a serious accident, you don't want to be treated in country, and you don't even want to know what evacuation to Thailand will cost.
Insurance against liability? I have never heard of that but because of the sad economic state of the country, you can probably afford to pay for any amount of injury you can inflict, out of pocket.