Low vacuum would mean a (mild) engine is not running optimal, or sometimes even struggling to stay running.
Could be from a lousy mixture, piston ring seal, vacuum leaks, ignition timing or cam timing.
Some peeps think 12-15" vacuum is healthy. I think around 20" is healthy for a mild engine.
With your cam I would expect it to be around 18-19".
But if all fails I will offer myself and take the car of your hands for €200,-.... This will save you from spending money on long term storage... :D
Have you ruled out the single (?) v-belt is possibly silently slipping on the crank pulley when running?
Is the power steeringpump rotating 'fairly' freely?