Most time of the year you can go surfing in Vietnam, although this is not the perfect surf country. If you have seen the movie Apocalypse Now, you probably remember the famous surfing part, which is actually filmed in the Philippines. You can hardly find any surf shops, but there are some tour operators who have surfing boards for hire.
The best surf season is between November and April, when the winter monsoon blows from the north. Every year there are typhoons forming in the South China Sea, who produce big wind swells. They also seem to disappear very fast, usually within a couple of days.
The original place where the American soldiers took their break is China Beach, a 30km long beach, which sometimes has waves over 2 meter. Be careful after heavy rain, when pollution can be washed into the sea.
In the main season there are good left-hand break waves at Bai Dai beach, just 27km south of Nha Trang. If you are a beginner, go to Mui Ne, with multiple breaks around the bay, including short right- and left-handers. Mui Ne is also a famous place for kite- and windsurfing. More to the south is Vung Tau, closer to Ho Chi Minh City, with one of the best waves in Vietnam when the sea conditions are good.
For much of the year the water is bathtub warm, with the temperature hovering between 25 C (dry season) and 30 C (wet season).
Be careful when you search for good waves off the beaten track. There are still remains from the Vietnam War, like Unexploded ordnance (UXO) around the countryside, especially in the area the the Demilitarised Zone (DMZ) north of Hue. Also look out for garbage, industrial pollution and other risks, specific at the big cities.