A perfect vacuum would be one in which the absolute
pressure would be zero (in any units).
That would normally occur when the amount of vacuum
was 29.92 inches of mercury (meaning that the absolute pressure was 29.92 inches of mercury below the ambient atmospheric pressure).
Just for your information, 29.92 inches of mercury is equivalent to 760 millimeters of mercury or 14.696 pounds per square inch.
(Visit me at www.air-dispersion.com