CO2 is Cumulative, Water Vapor is in a Cycle
I advice you to read the discussion below the article explaining water vapor’s interaction with CO2 emissions.
I know too little about this field to claim anything for certain, so it is better to make your own opinion on what is being discussed.
One of the quite relevant quotes I found was this one. It gives a possible explanation why water vapor alone doesn’t cause a runaway greenhouse effect. In other words you can’t use water vapor to amplify the effects of water vapor. You have to use CO2.
ONE: the CO2 we pump into the atmosphere stays there (except for the amount that partitions into the oceans and is absorbed by the terrestrial environment). Therefore atmospheric CO2concentrations rise cumulatively (and very very quickly now).
TWO: the water vapour that we pump into the atmosphere is a tiny supplement to the natural evaporative/precipitation cycle, and since this comes straight out of the lower atmosphere within a week or two it can (a) have only a very small effect and (b) caanot be cumulative.
THREE: AS the entire troposphere warms under the influence of cumulatively enhanced CO2concentrations, so the atmospheric water vapour concentration rises. This element (the water vapour feedback to enhanced greenhouse warming) is cumulative, and does provide a very significant supplement (a feedback or amplification) to the primary CO2-induced warming.
There is some more detailed discussion of amplification effect at NOAA. My impression is that a crucial element here is layering. The CO2 and water vapor doesn’t have the same effect at different layers and the layers affect each other. Hence that is why CO2 and water vapor are not direct substitutes for each other.