You should have a visa to enter Vietnam. But you can visit Phu Quoc Island with no visa for 30 days. Keep in mind you can enter Phu Quoc Island without a visa but cannot go to the mainland in Vietnam after landing at the this Island.
This paradisiacal Island is definitely worth visiting to spend your holidays. All travellers visiting Phu Quoc Island can stay there without a visa for up to 30 days. But for this, you should land directly on the Phu Quoc airport and stay within the Island.