Visa Requirement for Entry to Phu Quoc, then transit to Siem Reap, then back to Vietnam

Hello, I just booked flights for a trip to Vietnam in March and I'm trying to determine the correct visa/letter to get, but the fact that I'm flying directly to Phu Quoc is posing a challenge.

Here is my itinerary...
- Fly from Los Angeles to Seoul to Phu Quoc
- Stay in Phu Quoc for four days
- Fly from Phu Quoc to Saigon to Siem Reap, Cambodia (not staying in Saigon, just transferring)
- Stay in Siem Reap for two days
- Fly from Siem Reap to Saigon
- Stay in Saigon for two days
- Travel throughout Vietnam (total time from initial arrival in Phu Quoc: 20 days)
- Fly from Hanoi to Seoul to Los Angeles

Since Phu Quoc is not a major port, I cannot get a approval letter for a stamp there, but I learned that Phu Quoc has a visa exemption, so it sounds like the first leg of this trip shouldn't be an issue. Now I'm wondering the following...

- Should I get a multi-entry visa, since I will be flying into Saigon on my way to Siem Reap, then flying back from Siem Reap to Saigon?
- Should I get a single entry visa, since I will only be transferring through Saigon, so the official entry for visa purposes would be the second arrival for my stay in Saigon, after which I'll be staying in Vietnam nonstop until my final departure?

I didn't realize this would be so confusing when I originally purchased the tickets, so I sincerely appreciate your help!

Thank you!