Hello over there in 240vac land, I think I see a start of a solution
here.  Although I don't know the exact application, I'd offer the
following.  There are really several problems here.

1 - sense the vacuum level that exists
2 - sense or set the desired vacuum level
3 - output a signal that is related in a meaningful and useful way to
    these levels (like the difference)
4 - provide a device that will respond to the output signal and control
    an appropriate motor to produce the required vacuum
5 - connect the 'source of vacuum' to the player

Of course, the simple way is to listen to the music (1 & 2 & 3), make
your feet go on the pedals (4 & 5).  If you don't want to do that,
then there are a couple of approaches.  Of course, these ideas won't
work unless the player is complete with the matching pedals.

Connected to the pedals is a large reservoir bellows (maybe two, like
mine).  This provides #1 and #2 above.  The position of the moving
reservoir board is directly related to the pressure as long as it's not
closed (#1).  And there is a place where the board is positioned that
represents the desired vacuum level (#2).

You can buy an inexpensive device to control the speed of a woodworking
router (#4).  In the USA, they cost from $13, at Harbor Freight, to
$100 at the fancy woodworking stores.  A rotary knob controls the speed
of a universal motor from zero to 100% speed. You can even remove the
potentiometer and mount the box remotely if you need to.

It works fine on a vacuum cleaner motor of the type needed.  Just
replace the knob with an arm attached to the moving reservoir board
(#3).  Set the 'home' position of the shaft to collapse the bellows to
the desired point and tighten the screw that holds the arm in position.
You can tinker with the length of the arm and the attachment point and
angle on the bellows board to find the right combination of speed
change vs. vacuum change.  Or you can use a pulley and spring or other
simple device to increase the range of control if you need to.

I suppose you can figure out #5 yourself if you got this far.  (Hint:
either "Tee" into the hose or attach a nipple someplace out of sight.)

Very simple: no wiring,  no circuit design, no soldering, no expensive
sensors.

Regards
Craig Smith
Near Rochester, New York, USA