Sanya is on the southern tip of an island that is the southern most point in China. They bill themselves as the Hawaii of China (false advertising), attracting some warm weather seekers from farther north in China and some Russians.
Normally cruises heading north through Asia stop in Northern Vietnam at one of the beautiful places that allow a choice of just enjoying the area or heading into Hanoi for the day.
We got Sanya instead. Cruise lines are always looking for new places to visit. I think this was one of those places.
My guess is that it will be back to northern Vietnam in the future.