Visa for n& s vietnam

I will be entering ho chi minh on oct 31, 2017 from cambodia by air and from there go to hanoi on nov 7, 2017 with a tour group. Will I need separate visas for n & s vietnam. Also i will be proceeding to japan on nov 10 and returning to Hanoi airport on nov 13 for onward flight to boston, us. Do I have to get single entry visa if I do not go out of the airport on 13 for transit to next flight to boston, or would I need multiple entry?