Water vapour has a short residence time in the atmosphere, is self-limiting (excess water precipitates out as rain or snow), and is already at equilibrium given ambient conditions.
The one variable that could drive additional water vapour is CO2, which by warming the atmosphere further increases the ability to hold water, and the amount of water vapour present.
If you want to reduce water vapour as a greenhouse-gas component of the atmosphere, reduce CO2 levels.