My understanding of water vapour is that the maximum amount in the atmosphere is dependent on temperature (lets assume pressure is constant, for simplicity). If this amount is exceeded, the excess precipitates out and falls as rain. The only way it can be increased is if there is a rise in temperature.
So, the simple version of the question; for a 1 dec C in global average temperature, for whatever reason, how much additional warming would be caused by the extra water vapour in the atmosphere?
… and the compound version; given that water vapour is a greenhouse gas, and that warming oceans would give off CO2; also a greenhouse gas, how much warming in total, assuming enough time to reach equilibrium?
What other feedbacks (positive or negative) would modify this figure? … and by how much?