Crossing any border in that region, with a vehicle that hasn´t got your name in its papers might be a problem. I have done it, when the person who owns the vehicle has been present at the border, but haven´t done it with an authorization paper - the owner of the said vehicle said it would not work, but I cannot be 100% sure about this.
Vietnam, at least in December 2006, seemed to very much reject all foreign bikes, regardless of engine size. We tried with 115cc "chicken chasers", from Cambodia and Laos, because we also thought there´s a 175cc limit, and there wasnt. Would be even harder to get in with a rented vehicle, I guess.
But there are nice side roads in Cambodia leading to some smaller border stations, and they are fun rides, even if you wouldnt get into Vietnam. And if you´re only 1 or 2 people, who knows if its your lucky day (we were a gang of 8 falangs, I think they were put off also because of the size of our group).