The country has a long coastline, varied, complex climatic zones and generally a fair amount of rain. The best beaches are mostly in the southern half, and that's best Dec- May [dry, but hot and humid]. However, in that area it doesn't usually rain for long periods at any time, only showers, unless an infrequent typhoon hits the coast, so the beaches are basically OK all year round. The worst season is probably July -Oct [humidity, occasional rains, typhoons, and of course July-Aug is always a busy holiday period]. Oct - Dec has the best weather over the whole country for sightseeing, with clear skies and temperatures in the mid 20Cs. Plan ahead if you wish to travel during the Tet New Year festival [late January to early February] - it can be a problem getting accommodation and transport.
China Beach, between Da Nang and Hoi An [Central] Best May-Oct The most unspoilt Vietnam beach is the endless white sand of China Beach, where US troops landed during the 70's unpleasantness. The main beach is officially around the Non Nuoc Resort hotel but some locals argue that the real China Beach is at My Khe Beach about 5 km north of the hotel [both are near the tourist attraction of Marble Mountains]. Development is in on the way!
Nha Trang, Vietnam [Central] Best May-Oct Nha Trang, 'the Vietnamese Riviera', is a delightful little town of tranquil, friendly locals. It has 6 miles of splendid white sandy beach with azure water, backed by green hills and palm trees. The town has little noise or pollution because locals prefer walking or biking to motorbiking. You can find good Indian restaurants there and some of the best seafood restaurants in the country. Tran Phu [public beach] is inviting but beaches north of the town and on islands in the bay are a better option. For instance, Doc Let Beach [34 km north of Nha Trang], Dai Lanh Beach [37 km north], Bai Tru Beach in Bamboo Island are worth the trip for peace and quiet.
Mui Ne Beach, Mui Ne Bay, near Phan Thiet [South] Best Dec- May Mui Ne Bay is about 20mins [by bus] from Phan Thiet, which is 120 miles/198 km [a 3 hour drive] from Ho Chi Minh City. It has fabulous stretches of white sand, a scenic landscape and lovely locals. The climate is hot and dry almost all year, with an average temperature of about 27C. The good bits [e.g. Rang Beach] are the outlying beaches to the north, but the best is Mui NeBeach, an upcoming resort areaknown for its spectacular sand dunes. This is regarded as one of Vietnam's most beautiful beaches, but locals still needs to develop an attitude to cleanness, as rubbish piles up between resorts. The beach is also known for world-class wind/kite surfing [good wind almost guaranteed - though the sea-breeze can be over-the-top for sun bathing sometimes. See Mui Ne Beach Pictures. No hustlers/sellers. Lots of comfortable, reasonably priced hotels are available.
Hon Chong Beach, Ha Tien [South] Best Dec- May The coastline of Ha Tien is on the border with Cambodia [5m/9 km] and has some superb beaches, including the site of the French film L'amant [The Lover, 1995]. Hon Chong Beach is a very seductive beach with soft white sand and calm tropical water, while nearby Tien Cave has interesting rock formations. White sandy Duong Beach is also worth a try. The Hai Tac islets accessible by boat from Ha Tien offer some great diving for scuba freaks. The biggest of these islands, Hon Doc, has also some splendid white beaches. Or try Nghe Island [1 hour one way].
Bai Tam [Long Beach], Phu Quoc Island[South] Best Dec- May Phu Quoc Island in the Gulf of Thailand is Vietnam's largest island. Off Ha Tien, it's a hidden gem of Southeast Asia. The island has mountains, forests and superb sandy beaches perfect for nature lovers as well as sun-seekers. Some of the beaches can only be reached by hiking sandy paths - Bai Truong [Long Beach] on the west coast is apparently the best, where you can see sunset over the water. Bai Thom in the north and Bai Khem in the south are also popular. Major development plans are afoot.