What's the difference between "erotic massage" and a regular massage?

Men's massage in Vietnam (locally called "VIP massage") takes place in fully private rooms with a sauna and shower, and the customers are almost all men. You pay in two parts – a ticket plus a separate tip (required) – and opening hours are long, from about 9 a.m. to 3 a.m. Regular (non-adult) spas are used by everyone, often with large or shared rooms, charge only the menu price (many include the tip) and close around 10 p.m.