Phu Quoc is the most famous place in Vietnam that makes nuoc mam. The first press of aged nuoc mam is highly prized by connoisseurs, but almost impossible to use without first diluting it with water, as it is very salty and pungent. With the right ratio of water, lime juice, and sugar, nuoc mam is the perfect dipping sauce for much of Vietnamese cuisine. Knowing this, you'd be able to guess with good accuracy if a dish had Chinese origin, because the dish would be offered with a soy sauce-based dipping sauce instead.

P.S. Phu Quoc itself is a lovely place to enjoy the freshest seafood of all varieties on the cheap. Rent a motorbike and you would be able to see most of the island in just a few days.