re: Is Water Vapor a Greenhouse Gas? If not what are the implications?

What you are describing is the fact that water vapor (and corresponding clouds) both prevent heat from reaching the surface of the Earth and trap heat on the surface of the Earth. With less clouds and water vapor, more heat hits the Earth (higher highs) and more heat dissipates at night (lower lows). To understand the overall impact, you look at the average temperatures. As you noted, Mandeville has lower average temperatures than New Orleans and corresponding lower water vapor levels. While one example does not prove much of anything, at least in your example, water vapor appears to be a positive feedback loop to surface temperature as it appears, based on the information you posted, more heat is trapped by clouds (higher average surface temperature)than escapes from the lack of clouds. I am not making a broader statement about water vapor, as one example is certainly not enough, this is just analyzing the info you posted.