Mark is correct, but I can also appreciate that you are overwhelmed by contradictory possibilities. So, some input for you:

Vietnam is a large country with a large population, and defined by both a history of war/colonialism (flattening much of the historical architecture) and a history of a very strong will to industrialize and develop. This means two things for your trip: there is a LOT of Vietnam to see, and geographically most of it is going to be regular Vietnamese people living their regular lives, i.e. not much of interest to see for a tourist. You may have the obvious desire to do a long north-south motorcycle trip, but you'd spend the majority of it in traffic, seeing the same sort of industrial landscape.

Given that renting a scooter/bike is trivial in any part of the country - literally every backpacker hostel will have a deal with a local scooter shop to deliver one to the front door for you - I'd recommend keeping your riding to the areas where it's enjoyable, and doing the rest of the travel in the same way as the locals: by public transport, usually by air. Domestic air travel is dirt cheap, and there are comfortable long-distance sleeper buses for places without airports or train stations (e.g. Hanoi to Ha Giang).

Pick the pretty places to ride (Ha Giang loop, Sa Pa, Hue tombs / Hai Van Pass, etc.) and maximize the fun.