A reason to not include water vapor in a list of "greenhouse gases" is its short lifetime in the atmosphere. If you inject extra water vapor there, it rains down and exits the atmosphere in a matter of days, whereas methane etc stay for years --- humans emitting water vapor vs. CO2/etc to the atmosphere has very different relevance for the physics. Of course, the vapor does have effect on the radiation physics, and the fact that the equilibrium concentration depends on temperature leads to the positive feedbacks.