I agree to a point, I think water should boil in a vacuume, however the process of freezing a vapor is different than freezing a liquid. Then there is the process of sublimation to consider. In sublimation a solid (ice) is converted to a gas, water vapor without the benefit of the liquid state. The process is so fast that it may not have time to boil out all of its gasses. This should yeild a similar process to Ice Fog. In Ice Fog you go from many hundreds of degrees water vapor into an environment which can not hold a vapor. At temperatures below -30 F. water vapor must be expelled as water, but it never gets that far because at -30 F (or thereabouts) water vapor itself freezes directly into a solid and it creates six sided criystals which cause a refraction of light at angles of 22 degrees relative to a light source. This causes the effect known as a "sun dog" when you view a light directly through and almost otherwise invisible fog we call an ice fog. The crystals seem to align with each other either because of gravity or movement through the air (drag) or some combination thereof.

This also causes shafts of light to extend oposite from the source of light when you view that light from any angle other than straight on. It also seems to magnify dimly lit light sources making them seem brighter, and also more numerous when viewed straight on.

It is an interesting effect. One we need to study more.