As a liquid there is a certain amount of attraction between the acetic acid molecules that hold them together. Conversely, the lower the temperature the less energy is available to encourage the molecules to change their habits. At a fixed temperature the relationship between liquid and vapour states is given by the vapour pressure. If the temperature doesn't change then there can be no more vapour formed if the vapour and liquid are in equilibrium. But that doesn't mean that more liquid cannot be added.
Vapour pressure is independant of the relative amounts of the two states.
Imagine a damp cave. Adding more liquid water will not make the atmosphere in the cave any more damp, it may make bigger pools of water but the RH will not rise. In the same way the acetic acid continues to be formed but cannot attain vapour status, so it exists as a liquid within the film base. And hence a whole heap more acid can continue to be formed despite the fact that the atmosphere surrounding the film is saturated. I have seen films literally dripping with acetic acid.