Not likely a vacuum leak. Vacuum leaks are usually of a constant volume. This volume of unmetered air leans out the mixture to an unburnable state (when it causes problems).
The leak is a problem when its volume is significant compared with the quantity of air that was metered. This happens most easily at idle where the smallest volume of air is used. It happens least often at higher volumes especially full throttle where the largest volume of air is ingested.
There are a number of possible senarios for your problem, but since we started by discussing air flow/volume considerations lets look at one that could be your problem.
You must remember that in reality an internal combustion engine is really an air pump. Anything that reduces the efficiency of the ability to pump air reduces power. Think about what would happen if you restricted the size of the tailpipe to the diameter of a dime.
The way this happens in the modern car is to plug the catalytic convertor. As long as the airflow necessary to power the car flows comfortably through that dime sized hole there will be no reduction in power. This is the case at idle or part throttle - BTW the amount of air necessary to run a V8 at idle will pass through an 3mm hole (ask how I know), pretty small. In theory it would only need a slightly bigger exhaust pipe.
There are other possible reasons for such servere power loss and they include secondary ignition breakdowns (dist rotors shorted above a certain voltage, coils with low available voltage, dist caps with carbon traces and plug wires with low voltage shorts), fuel flow/pressure problems, throttle linkage problems and maybe some others.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician