Why isn't the volume decreasing?
The volume of the LIQUID water is insignificant compared to the volume of the container..........and so......
..........and so it may be ignored for a first approx. There is enuff liquid water remaining to saturate the entire volume of the container with water vapour. The point is that the volume of the CONTAINER is a CONSTANT, and their is enuff water to saturate the container at the given temperature.
Does this address your question, or have I missed your point?