My theory is, that even though water vapour traps heat...there's other places for it to go. Carbon dioxide, at least at almost all the temperatures that exist on natural earth, is always a gas. For all of the water pumped into the atmosphere, only a portion of it would remain as water vapour. The rest would work its way into oceans, etc.
Which brings up the point...the sea levels would still rise, and we'd still have SOME of the effects of global warming...just for different reasons.