Colin, as mentioned in the other post I did just that in the last two months (Vietnamese Minsk from Hanoi to Singapore). Whoever told you about individual travel in Cambodia / Siem Reap being impossible has no clue, I'm sorry. There are hundreds of independent travellers. Getting a Vietnamese plated bike from Vietnam to Laos or from Vietnam to Cambodia is also as easy as pie. They don't even ask for any motorcycle papers.
However, beware: it's not possible to get from Laos to Cambodia with a Vietnamese bike. I (and others I met) tried so at the border next to 4000 islands / Don Det. They will smile and won't care.
Also: getting a bike with Vietnamese plates to Thailand is tricky and takes time, but worked for me at the second attempt. If you need info on that, let me know.
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