Narrow roads, overwhelming traffic
Last year I traveled from the very south of Vietnam to Hanoi using public buses.
The roads are narrow and single lane with poor surfaces.
The traffic is heavy with scooters, bicycles and the occasional dray cart.
The population density is very high. Few nice open fast stretches of road.
A big capacity bike here would be wasted except for the mountain stretches out of Dalat and from Hanoi west.
It's 1145 kilometers from Hanoi to Ho Chi Minh. That's a long way on slow two lane crowded roads. Make sure you budget sufficient time.