"Andrew Dessler and colleagues from Texas A&M University in College Station confirmed that the heat-amplifying effect of water vapor is potent enough to double the climate warming caused by increased levels of carbon dioxide in the atmosphere."
http://www.sciencedaily.com/releases/2008/11/081117193013.htm
Water vapor will generally rise with warming, but where Dessler and others like him screw up is not properly estimating the distribution of that extra water vapor. If it is more concentrated (e.g. more storms, rainfall, etc) then that is a negative feedback and the modest CO2 warming will not be amplified, and maybe even reduced. If instead water vapor is evenly spread out (e.g. deserts become more moist), then there will be amplification. Models cannot pin it down since their predictive accuracy is nil. The recent history of atmospheric temperatures points to lower sensitivity (less amplification from water vapor)