Multiple or Singular Visa

Hello,

My friend and I (both Australian) fly into Da Nang on the 28th of July. We plan to fly to different parts of Vietnam through ought two weeks.
I was wondering if we had to get a multiple Visa as we will be going to multiple parts of Vietnam or if we just need the single visa?
I know that there is a cost for the collection of the Visa at the airport but I was curious on how much I have to pay if I were to book them today?
I have also been to Vietnam before and was just wondering if this makes it any easier for me?

Thank you!