No personal experience on this. But I´ve tried to enter Vietnam with a Thai-plated scooter, from Cambodia & Laos, a total 5 times, and this did not work (the persistent "under 175cc=OK" rumour seemed false, but this was in Dec-2006).

I´ve met people in Laos and Cambodia, who travelled with bikes bought from Vietnam, who claimed, that it should work ("it" being entering Vietnam with your own bike, that is known to be problematic – they surely must´ve already exited Vietnam with the bike at that time). But I guess there is no guarantee on anything.... and what I saw on those borders, it might fully depend on who is in charge that day. If you´ve got some time, and can try again, if you got turned back (we did that several times, find accommodation nearby, and return the next morning, hoping there´s new faces to deal with!) then I think you´d have a pretty good chance on a Vietnamese-regoed bike.

And only 1-2 people is better – we were a group of 8, definitely too large to ´fly below the radar unnoticed´.... in fact we almost made it once, got the border police permit and everything, but then unfortunately run into the damn customs ´NO MOTOOBIKE TO VIET NAAM´-guy.. I´d already had my Cambodian visa stamped out, but luckily they could still fix it! They often did not speak any English, BTW.

I think you go to Saigon and then you go to Lộc Ninh ( Binh Phuoc Province) Border Hoa Lư - Kratie . You pass easy to go Cambodia . Have a good luck .