Water vapor is an extremely powerful greenhouse gas - however, it's presence in the atmosphere is extremely variable and so it's very difficult to measure long-term trends versus just what the weather is doing today. That's why we use CO2 as our climate change marker. CO2 is nowhere near as powerful of a greenhouse gas, but it's stable in the atmosphere for centuries. That makes it easy to track trends, versus today's weather, and you're also tracking a variable that has long-term consequences. For the record, methane is another strong greenhouse gas but like CO2, it's concentration in the atmosphere is highly variable, so we don't tend to track it.