Thank you so much for your comprehensive reply. Vietnam is definitely a go (but not Phu Quoc, unfortunately), and one or two more nearby destinations are still a possibility, such as China/HK, Bali/Lombok and Kuala Lumpur. Per your suggestion, cash is probably the way to go for Vietnam, and I'll hold off on the FNCs for the other potential places I just mentioned. I actually have ten FNCs, but the first five expire early next year, so I need to plan now to use them.

For Hội An, I've read a lot of posts that it's been ruined by overtourism, so I wasn't planning to stay there overnight.