Do you know what a partial pressure is? What equilibrium vapor pressure is? If not I'll help you out...
The equilibrium vapor pressure of a substance is the pressure of that substance in a gas state when a liquid state is in equilibrium with a gas state at a particular temperature and total gas pressure. When the equilibrium vapor pressure is reached, no more of the liquid will evaporate. The ratio of the current vapor pressure to equilibrium vapor pressure for water is a well reported value in meteorology: the relative humidity. When the relative humidity reaches 100% the partial pressure of water reaches the equilibrium vapor pressure for water at that temperature. No more water can evaporate. Period. (If the equilibrium vapor pressure is lowered by for example lowering the temperature, water will condense out to make clouds, fog, rain, snow, etc.)
This means that we know the maximum amount of greenhouse effect that can occur from water as it very frequently saturates.
For carbon dioxide (at 0 C), the partial pressure of CO2 is 3500 kPa. The atmosphere at the surface has a pressure of 101 kPa, so CO2 is only going to saturate when its partial pressure is 35 times the current atmospheric pressure, or about 100,000 times the current CO2 levels.
Bottom line, water saturates frequently in the atmosphere, and CO2 is no where near saturation.