https://en.wikipedia.org/wiki/Water_vapor#Water_vapor_in_Ear...
On the other hand, what you said could be misunderstood to mean that human activities have no effect on the average water vapor concentrations, and that'd be inaccurate. The equilibrium point is determined in large part by global temperatures: the higher they are, the more water vapor is pulled into the atmosphere. And we do have a mechanism to increase atmospheric temperatures: CO2.
CO2 by itself is fairly limited in terms of temperatures: doubling its atmospheric concentrations only increases average global temperatures by about a degree, which is simple to calculate and more or less undisputed. But what really kills us (and where uncertainties creep in) is in the positive feedback loops, the biggest of which is water vapor. A degree increase in temperature from CO2 alone results in an additional couple degrees of increase in temperature due to the effects of increased water vapor.
Combine a couple of those positive feedback loops, and you've got a problem.
Methane clathrate guns aren't the same: the idea there is that we've not historically explored the relevant parameter landscape (oceanic acidity + temperatures). People hypothesize that, if we're not careful, we can stumble into a saddle point where a small delta propels us away from our current equilibrium and toward one with much higher mean surface temperature.
All of this is to say that water vapor feedbacks are well established, while a methane clathrate gun is just a speculative hypothesis.
On the contrary, deforestation has a huge effect on the water cycle. Most people don't realize that if you're more than ~200 km inland most of your precipitation comes from transpiration by plants upwind instead of evaporation.
How do we know? Oxygen isotopes. Basically, the differential fractionation of oxygen (and hydrogen) isotopes in water during evaporation vs. transpiration allows you to trace where your rainfall comes from. http://www.nature.com/nature/journal/v496/n7445/abs/nature11...
Also, don't cut down forests thinking it'll fix global warming. Overall they have a net cooling effect. What you'll do instead is create deserts downwind.