Visa Required?

Hi, i am planning to travel to vietnam via cambodia, where i will cross the border at Ha Tien, then board a ferry to Phu Quoc which i understand does not require a visa. I will then fly to Bangkok from Phu Quoc, via Ho Chi Minh City. My question is, i plan to book two flights to get to bangkok. One to fly to HCMC and another from HCM city on the same day. But by doing this, i will have to pass through immigration at the domestic terminal and then check-in again for my flight to Bangkok. I have no intention of leaving the airport and i should have 2 hours between my connecting flights.

Will i require a Visa for Vietnam if I don't intend on leaving the airport but will need to check-in for my new flight?

Also, do you know if I would need a visa for entering the border at Ha Tien?

Please note i carry an Irish Passport.

Many thanks .