Can I pay with Thai baht on Phu Quoc and in Vietnam?

No, Thai baht is not accepted for payment in Vietnam in general and on Phu Quoc in particular. Officially, all payments here are only in dong, and in practice, almost everywhere and always prices and payments are in dong. The only exception is travel agencies that cater to tour tourists, and they semi-legally quote and may accept payment in dollars, but certainly not in Thai baht. For more details on everything you need to know about money and currency in Vietnam, read here - https://nashaplaneta.net/asia/vietnam/money

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Follow the link, all these questions are discussed in the chat - https://t.me/vietnam_travel_chat