The simplest way to make a vacuum gauge is to make a manometer out of
clear plastic tubing stapled (with round staples) to a piece of wooden
board followed by half filling the tube with water.  The vacuum (or,
indeed, the pressure) can then be read off directly, unless you are
a metric freak, in inches of water.

However there is a more suitable alternative here in the UK and that
is to search the junk markets for an early WW2 airspeed indicator
calibrated in Knots.  These usually cost less than 5 pounds ($7.50)
and go up to something like 45 inches water column or more on the
second revolution of the pointer.  They can easily be recalibrated in
inches of water using the homemade manometer and exercising your lungs.
The required vacuums can then be annotated on a round annular disc of
cardboard pasted onto the glass of the instrument.

David Snelling
Isle of Man