The strange thing is: if there would be no pressure in the straw, nothing could float up inside it. Buoyance is nothing more then the resulting force of pressure differences.

AT the top of the water in the straw we most likely won't have vacuum, just low pressure. And exactly speaking the vacuum doesn't hold the water up (e.g. is not applying some upward force). It is the air pressure of the surrounding atmosphere pressing the water into the straw, because the low pressure in the straw can't compensate.

Thats why the water level in the straw rises, as long as the weight of the water column equals the pressure difference between the surrounding air pressure and the lower pressure at the top of the straw.

At least that's what I guess smile

This leads to the following conclusion:
If you have a perfect vacuum in the straw (no air pressure) there is a maximum height to wich we can lift the water along with the straw. This height is determined by the air pressure.

Since air pressure is 1 bar = 100,000 pa = 100.000 N/mē.
A "straw" can hold at most a ~10m column of water, after that lifting the straw wont lift the water anymore.

Actually, for the same reason this is also the maximum lifting height of any liquid achievable by a suction pump.