And how many vac pumps have you changed in your vast experience as an engineering expert.
In my experience there was no way before the plug could you tell when a pump was to fail. There are no indications piour to a failure. They work and then they stopped. That simple.
Now we have a port where the vanes can be measured. But as the beast is what they are they can run a life out or they can still fail. Is really in the lap of the gods to what you can get. If you don't won't a failure put a wet pump on. They all but last for ever.