Plus the water vapor rises more easily than CO2. A moloecule with two hydrogen atoms and one oxygen atom is considerebly lighter than O2 or N2 (oxygen and nitrogen respectively) and its part of the reason why moisture laden air rises and convection works. (heat transfer is the other part of it).
Water also has a high specific heat, meaning it can hold more thermal energy, per unit of its weight. That's why our oceans make such a good heat sink. Water can absorb heat without its own temperature rising as much as most other substances, due to its hydrogen bonds.
All models of GHG-induced global warming include water vapor feedback. If they didn't, they wouldn't work.