Vacuum = nothing. Nothing cant hold anything.
any pressure below 0 guage pressure is refered to as a vacuum.http://en.wikipedia.org/wiki/Vacuum_gauge
there are low vacuums , medium vacuums , high vacuums
and ultra high vacuums and extremely high vacuums.http://en.wikipedia.org/wiki/Vacuum
And buoyancy is the result of pressure differences between the top and the bottom side of the object.
and the pressure is the result of density.
so density is the true reason for buoyancy. http://en.wikipedia.org/wiki/Buoyancy
This picture might give you an idea what I mean:
red dot at the top: pressure of the liquid at top, force downward.
red dots at the sides: equal pressures, canceling each other out.
red dot at the bottom: pressure of all the liquid above it, this pressure is more then on the upside of the object.
Resulting in an upward force.
you are refering to a open system that is exposed to the atmospheres pressures , not a system that is exposed to a vacuum.
in a open system pressures are distributed in layers
and the only reason that water in a open system ever exerts pressure outward or upward is due to the weight of water
in a closed system pressure is distributed equally in all directions.
you continue with outer space which is also an open system
there is a difference between open and closed systems.
1) open systems are able to interact with atmospheric pressures
2) closed systems are not.
the straw system is a closed system and its bottom is the water pressure that is in a open system.
it should be clear to you that this straw in a glass system is a combination of both open and closed systems.
something you should try.
get a glass bottle fill it with water.
fill a pot with water , now turn the bottle upside down into the water in the pot.
now lift the bottle slowly out of the water observing the top of the bottle very closely.
the bottle can come completely above the surface of the
water in the pot , yet the water in the pot rises up to the bottle as you lift it.
still no water comes out of the bottle.
even though it is clear that there is not enough water pressure along the mouth of the bottle that could be supplied by the water pressures in the water in the pot
to support the weight of the water in the bottle.
so wheres the pressure that is holding the water in
remember in order to have water pressure you must also
have water at a elevation.
but theres no water higher than the mouth of the bottle.
so is it just atmospheric pressure that holds the water
in the bottle?
but if I continue to lift the bottle the water comes out.
so it cant be just atmospheric pressures holding the water in the bottle.
its the vacuum at the top of the bottle that holds the
water in the bottle , not the atmospheric pressure pressing down on the water in the pot.
I used a 1 gallon glass bottle when I tried this.