Vacuum = nothing. Nothing cant hold anything.
Try holding a board by NOT attaching it to the wall
And water is an incompressible fluid, there is virtually no difference in the density of the liquid.
Anyway: a minimal density difference in the water would be THE RESULT of pressure compressing the water.
If you were right the water in the straw would have no density gradient. Nothing could float there.
The water is hold by the air pressure, hence the maximum lifting height of suction pumps.
And buoyancy is the result of pressure differences between the top and the bottom side of the object.
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.
There has to be no density difference whatsoever.
Ah, found it: http://en.wikipedia.org/wiki/Suction
"Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not innately attract matter. Dust being "sucked" into a vacuum cleaner is actually being pushed in by the higher pressure air on the outside of the cleaner.
The higher pressure of the surrounding fluid can push matter into a vacuum but a vacuum cannot attract matter."
This should be obvious. If vacuum had some magical attractive force, what would happen to the ISS? The realy BIG vacuum of outer space sucking in one direction, the tiny vacuum in direction of earth sucking with less force in the other direction....