why does water boil in a vacuum?

It does not boil from heat but from the free and eratic movement of its molecules that are free from the binding constraints of pressure and gravity of the earth. The boil is the energetic movement of the water molecules.

Well, it kind of "Boils", but what is happening really is that is actually evaporating.

Once you reach the vapor pressure of the liquid, it starts to evaporate without applying heat.

This applies to ANY liquid, it just so happens that water is most common, but if you put acetone in a container, it will "boil" or evaporate faster than water, because it's vapor pressure is lower than water, but if you put Oil, it will require a LOT of heat in order to evaporate, and what will happen is that you will burn first.

No atmospheric pressure, so the water molecules don't need a lot of energy to be able to break free and thus boil.

That's why.