Thanks @tomcarpenter. Yes, you can do it by bicycle too but it'll take time and energy :)

If you drive a motorbike under 50cc, you should be cover. Unfortunately above, regarding travel insurance, for most of them, you cannot be cover because they only recognise Vietnamese motorbike licence and you cannot have it unless you're Vietnamese or resident.
I am not a specialist of this subject but I found this article about it. https://vietnammotorbikerental.com/vietnam-motorbike-driving-license

Travel by bike is definitely silly, risky and dangerous, in the same time it is a fantastic way to discover the country, just need to be really careful, wear the good equipment, understand the traffic...