It seems like Phú Quốc is two islands. There are the fancy resorts, restaurants, and beaches that the tourists confine themselves to. But there is also the Vietnamese city of Dương Đông that the locals live in.
Dương Đông is quite a large, congested, noisy, dirty, and sprawling place. It has a harbor full of fishing boats and temples dedicated to fishermen. There is an overpriced tourist night market selling expensive seafood, but there are also restaurants for Vietnamese people selling good, cheap seafood.
Beaches are boring and all sort of look the same, so in some ways I found Dương Đông to be the most interesting part of the island.