I suppose the thing about water vapour is we're not burning vast amounts of natural hydrogen, thereby adding to the total water vapour present in the atmosphere.
Here's a more interesting questions, does the natural warming of the sun, thereby melting ice, causing more water vapour, causing positive feedback (more water vapour -> more greenhouse gas -> higher temperatures -> more ice melts) have more of an impact than gradually rising atmospheric CO_2 levels? Don't bother trying to answer that one, nobody knows. The one thing I'm sure of is it's been getting hotter for the last 25 years, and till we have a 2nd planet I find that unsettling.