"When Al Gore was asked what are the greenhouse gasses, why did he leave out water vapor? "

Alas! I am not a mind reader! But I suppose you are, so you will come up with some esoteric conspiracy involving the colour of his underpants...

Also, at a layman's guess, the amount of water vapour is on average a fairly constant percentage world wide.

And, ya know, mate, when it gets too high a percentage, it's called Rain!
:-)