Water vapor is not able to sustain warming on it's own -- it only contributes after carbon dioxide levels are raised. It is a feedback loop essentially. The reason for this is the albedo/shading of clouds is nearly self-canceling. But when the carbon dioxide levels are raised, and the temperatures get higher, we then get more evaporation and there is more moisture in the atmosphere.
We *are* the primary cause for the current warming of the climate. That is what virtually all the climate science concludes. If the carbon increase was carbon 13, it would be from the volcanoes, and if it was carbon 14 it would be from "new" carbon - but it is carbon 12 which can only come from "old" plant carbon.
Watch these programs with Dr. Richard Alley and many others to find out about the science:
Earth: The Operators' Manual | Earth: The Operators' Manual