Water vapor is a GHG. It's nearly all in the lower troposphere of course.
It's a "forcing factor" I mentioned above. Stipulate that our climate heats up by 0.1°C, that means more water vapor can be present (will be present). More of that means more heat retention, which means more water vapor, and so on. This is on way a small effect can compound or spiral into a large effect.
Clouds are more complex factors as they both keep heat in and increase our albedo reflecting more "heat" into space. This is a simple step into the complexity of climate models.