and condenses at most earth temperatures.
This is the key point. You can't just pump more and more water vapor into the atmosphere. There's an upper limit; once you hit the limit, it condenses and falls out as rain. So you won't get runaway warming just from H2O.
But there is a secondary effect that should be noted: hotter air can hold more water vapor. So as the atmosphere warms from CO2, it can hold more water, which is a greenhouse gas, and it warms even more. It's not a feedback effect, but it is an amplification effect.