Water vapor does contribute to global warming, indirectly. From what I know, it IS a factor in the models. Water vapor can actually help cool the environment if it gets made into clouds, but that's a seperate process.
Part of the confusion in regards to water vapor vs. carbon dioxide is that it's not really possible to say "This amount of water vapor is responsible for this amount of warming" because the effects arent additive. (This is one of the reasons some people think that human contributions to global climate change arent' as bad as established science says.)
Every gallon of gas creates about 8lbs of water, so the effects of water vapor ARE taken into account in the climate models.
Now, the way greenhouse gasses work is because they basically absorb heat. Water vapor absorbs heat, which is one of the reasons it's good at cooling.
I know, that sounds conflicting, but a basic explanation:
Basically, the greenhouse effect is when gasses in the atmosphere absorb reflected heat from the sun, trapping it in the atmosphere, warming it.
In smaller environments, like after a rainstorm, or using a swamp cooler, the water in the air absorbs heat by evaporation, making the area cooler. The heat has to go somewhere
