Havent ridden in Vietnam, but I suspect it´ll be roughly the same as in most countries in Asia: if you have an accident, no matter who caused it, and you´re a falang --> you pay. If you crash your rental vehicle because of whatever --> you pay. If your rental bike is stolen --> you pay, even if it later turns out, that the thieves were the same people, who rented it!!
You should get a comprehensive travel insurance policy for yourself, before you travel, and make absolutely sure, that it will cover you even if you get smashed up riding a motorcycle. And that it will also cover a medical evacuation to Bangkok or elsewhere in the worst-case scenario (that would be hugely expensive, and you could be denied such a service, unless you have the proper documents to prove your insurance will cover). This kind of insurance may cost a bit, but I wouldnt leave home without it.