Blue water is not so common in the Sea of China (coast of Vietnam).
For what you are asking, Mui Ne Beach near Phan Thiet would be the choice.
The beach is great, not many hagglers at all. A very laid back relaxed place. The water can be blue, subject to the time of year. No surf really, but popular with para-sailing and windsurfers.
As for nightlife... not a great deal. In Mui Ne itself, there are a couple of nightclubs. Fairly small... Maybe 60 to 80 people on a busy night. Beers are cheap and restaurants vary from $2 to $40 dinners.
A 15 minute taxi ride (about $4) into Phan Thiet will get you into the town centre where there are quit a variety of nightclubs.
You can take a bus (about $4 to $6) from Saigon which they say is 4 hours away, but it can be as many as six subject to the traffic. Alternatively, you can take the train to Phan Thiet, then taxi to Mui Ne. Another is to talk to your hotel staff that might find you a seat on a minibus which is about the same price as the bus, but normally takes no more than 3 hours to commute, and will take you right to the door of your accommodation.
Beach front 4 and 5 star resorts in Mui Ne start at around $40 per room per night subject to the time of year, with some exceeding $100.
There are budget hotels back from the beach from as little as $10 a night per person.
From Mui Ne, you can go on jeep tours or rent a motorcycle and visit the great sand dunes about 30 minutes to the north. (You can go sliding down the dunes with many kids on site happy to rent you boogie boards to do it.)
There are also a number of tours you can get a seat on for as little as $10 where you can visit Dragon Fruit plantations, salt farms, shrimp and fish farms and also out to some ancient temples.