I have never been to Vietnam but I spent motorcycling around other Asian countries, including nearby Thailand. I asked around this as well because Vietnam is on my to-do list also:
Vietnam is growing like weeds and it's not as backwards as it looks on TV shows.
No problem with paved roads. If you want dirt and water crossings, you can have that too - just like I can have that here in Maryland and Virginia if I get off the beaten path and big roads.
The biggest issue with Vietnam is that it's a crowded country. There is traffic everywhere.
From the research I've done, it would be a mistake to end in Hanoi. The interesting bits are in border areas with China/Laos. That is where you will find spots that haven't been overrun by tourism, less developed, and with twisty mountain roads and maybe some stream crossings and dirt if you want.