If its an older device then it probably is a voltage out.
Sounds like you need an op amp, an amplifier with a positive gain will decrease the sensors range, and a negative gain with increase it, youcan also add an offset pot to shift the whole scale up and down, its pretty tricky to do.
Messing with the vac sensor by the way isnt a great way of increasing performance, some aftermarket gizmo's claim to improve performance, often false claims, the pcm uses the vac input on a alpha n system to calculate the airflow and therefor calculate the amount of fuel, ie injector on time, a closed loop system which nearly all are monitors the exhaust gas and makes continual corrections, this would cancel out any adjustment you make to the vac signal, until the adjustment gets more than 10% then you'll set a too rich or too lean dtc code.