I guess I don't understand your query… The reality is that you can't have water in a vacuum. I guess truthfully… you can't really have a vacuum. So maybe you are just talking a low pressure area… in which case the glass is still full of air.
it's a theoretical query plain and simple, same as all the others.
even in a low pressure area, i don't think the space is filled with air.
if scientists say they can create a vacuum that holds only a limited number of atoms what takes up the rest of the space?
or maybe the question should be how many atoms does a space need to be called air.
ok yes there is no such thing as a perfect vacuum, but lets say you could achieve the same type of vacuum as in space with just a few hydrogen atoms per cubic metre,
the water's not in the vacuum, it's to one side of it in the glass.
you'd have the glass with the water on the bottom and the space vacuum on the top with just a few hydrogen atoms followed by a seal.
so now there'd be no air in it.. just a space vacuum.
i think what would happen is the glass would now be filled fully with water alone,
because the water atoms would expand to take up the empty space left by the vacuum.