It's both, actually. Water vapor is positive no matter what. Clouds can be both positive and negative at the same time, depending on cloud type, altitude etc.
This is one of the things that are hard to model. Get this wrong and you can swing from a mere +1 global warming (what CO2 alone would do in 100 years) to +3 or more.
Edit: this is a genuine question. The last time I read about it the result was inconclusive whether it caused or hindered warming.