In short
Phú Quốc sits in the Gulf of Thailand, closer to Cambodia than to the Vietnamese mainland. The northern half is a national park; the west coast holds the beaches and the resorts.
The island has its own visa exemption: thirty days with no formalities if you arrive directly from abroad and do not leave it.
What it really costs
Hotel development has been rapid and the south of the island is now on international prices. The north and east stay far cheaper.
Phú Quốc pepper and locally made fish sauce are the two traditional industries. The night markets in Dương Đông feed you for a few euros.
What nobody tells you
The visa exemption lapses the moment you reach the mainland. A return trip to Ho Chi Minh City from Phú Quốc requires a valid visa, and plenty of people find that out at the gate.
Construction is heavy and constant on the west coast. Some beaches presented as idyllic sit alongside building sites.
The rainy season, May to October, brings daily storms and rough seas. Sao Beach and the An Thới islands are far less usable then.