odds are that you would lose
The original statement:
"CO2 pound for pound [or in terms of liquid or gas volume] is not a substance which is as effective greenhouse agent as H2O"
According to that link I gave you, there's 5 times as much water vapor than CO2 by mass, and the greenhouse effect is somewhere between 7 times as much from water vapor, to 1.5 times as much compared to CO2.
By volume, one of the terms you wrote, C02 is certainly more effective, and by mass it is probably more effective.