I just got back from Sapa with some friends and it was awesome! The weather was perfect, and the locals said that November and December are the best months for “cloud hunting”. So we hiked up Mt.Fansipan, the highest mountain in Indochina and it was totally worth it. The sea of clouds was absolutely beautiful.
There's a village at the foot of Fansipan called May, which i was told that it mean “cloud village”, where we met some really cool locals. They showed us around and told us all about their culture and traditions. Plus, the food was amazing!
So yeah, so ahead, early December is a great time to visit Sa Pa. Hope this help you!