Vietnam forum

In short, the weather is erratic, but most people would tell you that December is a good time to walk around Hoi'an but not great for beach weather - Phu Quoc will average much less rainfall and about 5C hotter. I also found Hoi'an very touristy so looking for somewhere like Phu Quoc but not so touristy would put Hoi'an off the mark in my opinion - though there is plenty to do around there and I'm not saying that I don't recommend the region, I quite enjoy Da Nang/Hoi'an area.

I think you can easily fill up a couple days in PQ. My girlfriend and I spent something like 6 nights at Thong Loi (sadly closed we hear) up in a quiet bay and just loved it. A day going into town was nice and a day trip that took us boating to some snorkeling and other beaches, opportunities to rent motorbikes, and Long Beach is lively if you're looking for more to do. I'd really recommend the area around Bo Resort and Mango Bay though, that's where Thong Loi was (though Bo doesn't seem to have any beach-side bungalows).

I've known people who've been disappointed with Nha Trang, but can't speak from personal experience on that one - maybe someone can offer that as a viable option. Really, the weather is splendid in Dec-Feb around the Delta so with only two weeks I'd consider HCMC and a few days exploring the delta all the way to Rach Gai and taking a boat over to PQ, then just arrange to fly back from PQ to HCMC. Would be a splendid holiday and wouldn't have you bouncing about the country so much. You can save central Vietnam for another trip - it deserves two weeks itself in my opinion.

We didn't use a tour, just took the regular minbuses from HCMC to Can Tho and explored around there. I'm not sure if tours allow you to go all the way to Rach Gai and then go on your own to Phu Quoc. Maybe someone else can give better advice. Depending on how you want to explore you could just head into Can Tho and explore around there a couple nights and then head on to Rach Gai on your own, with some help from the front desk at the place your staying I doubt it'd be too difficult.