I was there twice in late December and the weather was awful both times. Windstorms that made it impossible to be outside. Chilly temperatures.

It seems that November will be similar, unfortunately:“In general, autumn travel in Nha Trang is not recommended. Though it is not cold, heavy rain, tropical storm and typhoon may affect most travel plan. Travelers planning to visit the area from September till December should keep your eyes on the local weather forecast.”