US nationals need an e-visa to enter mainland Vietnam. They don't need any kind of visa to enter Phu Quoc provided that they arrive directly from overseas.
US nationals require a visa to enter mainland Vietnam. They don't require one to enter Phu Quoc provided they enter directly from overseas and leave to go overseas.
But you have to enter Phu Quoc from overseas and leave Phu Quoc to go overseas - you can't go from Phu Quoc to mainland Vietnam unless you have an e-visa already as a US national.
I don't know if you're just genuinely stupid or just playing the role here. Either way, there's no such thing as an EU passport and not all countries which are members of the organisation known as the EU get 30 day visa exempt entries to Thailand.