Vietnam has a lot of beaches, and many travelers spend the last 3-5 days on the beach before returning home.
Hoi An in the central Vietnam has long white beaches in combination with a historical very interesting city, and the best time to visit the beaches of Hoi An is during dry season from february to august.
Nha Thrang is located further south in Vietnam, and thus has sea temperatures a little higher than in Hoi An. Air temperature is in average in the late 20’s most of the year. The coastline of Nha Trang is more exposed to the south-east, meaning precipitation is much lower here than in Hoi An. Dry season is still the best time to visit Nha Trang, and like Hoi An this is from february to august.
The beaches at Mui Ne and Phan Thiet has even lower precipitation, so here you will also find sand dunes. As with the beaches further north, the peak of rainy season spans a couple of months.
The island Phu Quoc has stable high water temperatures year round, and also high air temperatures. Rainy season is during summer, so the best time to visit Phu Quoc is from november to april.