The air descends on the lee side because, notwithstanding the water vapour changes it is still colder and more dense than the surrounding air mass.
Actually, air with water vapour in the mix is less dense than dry air for a given T and P. This is because water vapour has about half the density of dry air.
However, at max, there will be only 4% of water vapour in any met air mass, so the reduced density from the water vapour is tiny compared with any effect of changes in temp or pressure.
Dick W