For Saigon and Hanoi, I suggest that you book in the center of town since they’re both very extensive cities and in Vietnam,public transport is almost nonexistent (there are no metro lines and buses are always jam-packed).

Other cities are more “livable”; so if walking for 20-30 minutes isn’t a problem for you, you won’t be forced to book in the middle of the city.

Vietnam is a pretty safe country. The only problem that you might come across is theft from rooms, especially in cheap hostels where the locks aren’t all that great. So, if you don’t trust the locks, I recommend that you don’t leave valuable objects in your room!