My information may not be fully up-to-date anymore, but 3 years ago I was in a group, who tried to enter with 115cc scooters a total of 5 times on 4 different borders in Cambodia & Laos (all 3 international ones in Cambodia, plus Laobao in Laos). No success, but it was a fun trip nevertheless! Our ride report from that trip is at gt-rider dot com Vietnam-forum.
There are many rumours about this topic circulating the net, usually they are false. Like the "under 175cc is OK" - at least in 2006, this was simply incorrect information. Still you often hear this claim.
Lately, it seems that some have been able to arrange entry into Vietnam on big bikes (again there´s more info on the forum I mentioned), but this has involved a large amount of paperwork, a guide, and considerable costs, too - looks a lot similar to entering China the ´official´ way, so not easy, or cheap.
It might give you a better chance to get lucky at the border, if you´re a very small group. We actually came very close sometimes, but our group was too big (drew too much attention including the customs officers, who knew the rules, and always managed to stop us, before we were through!)
There were also offerings to smuggle the vehicles in, but I would not recommend this, as you may end up in trouble later on inside the country, or at the exit border.