Sound advice above, I think large, temporarily imported bikes are banned in Vietnam and the odd person has got lucky at the border and managed to get in with one. The small scooters that the locals use are most suitable even for trips the length of the country, traffic density and Asian driving means you would rarely get into top gear on a big bike so I would get one out there and save yourself the hassle and expense.
If it's in the Touratech catalogue then you don't need it
"You've got to think for yourselves! You're all individuals!"-Brian of Nazareth