Many travellers have previously posted that Vietnam does not allow foreign registered bikes to be brought in. That would seem to be a showstopper for your plans?
I can confirm that you can take a rented Vietnamese bike into Cambodia and Laos. No idea about Thailand.
If you rent, you will have to pay a substantial deposit, probably the whole value of the bike. That's because you need to take the original registration card, which would open the door for you to sell the bike.
cheers
Colin (Sydney)
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