I´ve heard it can be done on a Vietnam-regoed bike. But it may be a whole different story on any other bike.
And I believe you might actually be able to sneak in at some border, while the customs are not looking. I´ve been trying to cross there a total of 5 times, from 4 different border stations in Cambodia & Laos in December 2006, and had we not been a group of 8 big hairy falangs, that´s what we probably would have tried (our bikes had Thai plates). Some of the borders we went to, had lots of traffic going both ways, surely a small group like 2 or 3 people could have gotten through. Our group was too big, and we didnt want to split it, in case some had got in, and others had not. And we were also offered to smuggle our bikes inside a truck through the border, but didnt go for that either.
I´d advice to think carefully, before you decide to do something like that. It means, that you have brought your vehicle illegally into the country, so there are no guarantees, that you would not get into trouble either trying it, or later on inside Vietnam. Remember your foreign plates may also attract the attention of the authorities.
I would not say, that Vietnam is now possible - or at least it does not seem to be possible in the same way than, say, Thailand, Cambodia or Laos.