Phu Quoc Island, from the Gulf of Thailand, is Vietnam's largest island

https://potiqueviet1971.doodlekit.com/blog/entry/16328770/seven-good-reasons-which-make-nha-trang-come-to-be-one-of-the-most-touristy-spot-in-vietnam

Phu Quoc Island, while in the Gulf of Thailand, is Vietnam's largest island, providing mountains, jungles and nice sandy beaches; it appeals to mother nature enthusiasts and Sunlight worshipers. The furthest west