Water vapor is not a gas. It is no more a gas than fog is. It is water.
...and CO^2 isn't a gas; it's dry ice?
http://iaspub.epa.gov/trs/trs_proc_qry.alphabet?p_term_nm=W
WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. While humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation
Water vapor IS a gas; Fog is created when water vapor is condensed into droplets, much the same as rain.
Relative humidity is the measure of how much water vapor is present under current conditions, compared to how much water vapor the atmosphere could hold under the same conditions. At 100% RH, the air is saturated; if the temp drops, or more water vapor is added, it condenses
to liquid either as dew, fog, cloud, etc.; or, under some conditions, skips the liquid phase, and condenses directly as ice crystals.