Type of visa needed

Hello.

I will be traveling on a multi-country trip  this summer. I will arrive first in Vietnam and then travel to other  countries. I have booked a round-trip flight to and from Hanoi, and will be booking other legs individually while in Asia.

On return, I will be arriving at 8:00pm in Hanoi and leaving from Hanoi the same night at 11:00pm. I will not be leaving the airport. Will I need a multiple entry visa for this trip, or will a single entry do?

Also, my arrival and departure from Hanoi will be about 2 months from each  other. Can I still do a one month visa, since I am only leaving the  airport on the first part of my trip?

Thank you for your help.