Regarding the vacuum above the mercury column
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There is a question: atmospheric pressure is equal to 760 mm of mercury column, and there should be a vacuum above that column. But shouldn’t liquid vaporize in a vacuum? How can this be explained?
Then it’s not a complete vacuum. For example, take mercury as you mentioned; its saturated vapor pressure is very low, so the vapor pressure above mercury can only be even lower, so low that we can ignore its pressure
For vaporization, there is a vapor pressure; once equilibrium is reached, vaporization stops. At 20 degrees, the vapor pressure of mercury is 0.001201 mm of mercury column, which is negligible for production pressures.