We understand that we have friends who have dietary restrictions, so we want everyone to be prepared. Phu Quoc has a night market that serves plenty of cheap, but delicious eats. Even though you might not know what the food is, here are some Vietnamese words to look out for.
Pork thịt heo, thịt lợn
Chicken thịt gà
Beef thịt bò
Fish cá
Vegetarian ăn chay
Fresh Milk Sữa tươi
https://www.tasteatlas.com/100-most-popular-dishes-in-vietnam
This site lists more foods than you'll probably be able to try the whole week you're visiting, but it's a good idea to see the variety of foods Vietnam has to offer. It also list ingredients that can be used in each dish.
https://phuquoctrip.com/en/blog/10-must-try-phu-quoc-foods-where-to-find-them.html
John's tour provides some must try foods along with other travel tips.