are you now talking about getting big bikes imported & registered into Vietnam? That is an entirely different story than getting your own, foreign-registered bike thru the border, as a tourist.
My own experience is that the latter is not possible (unless MAYBE, if you have residency, speak vietnamese, know your way around their bureaucracy exactly, and have lots&lots of time and patience) It even seems the "under 175cc = ok" rule, if there ever was one, is no more.
Them joining the WTO might change their big bike imports, but will it change their policies for tourists with vehicles at their borders, i dont thinks so... not very soon anyway. I for one hope that Im wrong, it´d be a very interesting place to visit.