There is no climate model or climate textbook that does not discuss the role water vapor plays in the Greenhouse Effect. It is the strongest Greenhouse gas, contributing 66% to 85% to the overall effect when you include clouds. It is however, not considered as a climate "forcing" because the amount of H2O in the air varies basically as a function of temperature. If you artificially increase the level of H2O in the air, it rains out immediately (in terms of climate response times), similarly, due to the abundance of sea surface, if you somehow removed water from the air it would quickly be replaced through evaporation.
CO2 put into the air by burning fossil fuels, on the other hand, has an atmospheric lifetime of centuries before natural sinks will significantly absorb any excess from the air. This is plenty of time to have substantial and even longer lasting effects of the climate system.