VISA 3 entrees

Hello. We are a Mexican family (father, mother & 2 adult daughters) visiting Vietnam as tourists, with 3 entrees:
October 14th, 2018 NHA TRANG, with Norwegian Cruise.
October 15th, 2018 HO CHI MINH CITY (PHU MY) with Norwegian Cruise.
November 19th. HANOI just a 1 night stopover on our flight from Siem Reap to Tokyo Haneda with Vietnam Airlines

We all ready bought:
-Norwegian Cruise tickets.
-Flight from Siem Reap to Tokyo, 1 night stopover in Hanoi.
-Hotel in Hanoi 1 night. November 19th, 2018.

What kind of visa do we need and when do we have to apply?
Apply on line?
Requirements?

Thank you very much for your time.

Teresita