Don't be too hasty in blaming the vac pump and its setting for the low vacuum. Just over a year ago I had major autopilot work done on my C210M. On return to service I noticed that the vacuum was reading low. I had the pump adjusted to its maximum which brought the reading into the green but on the low side. I assumed the vac pump was on its last legs.
All went well for 300 hours with no changes to the vac reading and with the AH and DG functioning properly. Then one morning on startup the AH wouldn't strut its stuff. As best I could, I shone a torch on the vac lines looking for cracks or disconnects. I also had a grope around the fittings going into the instruments. All seemed well. Assuming I had an expensive AH replacement problem I took it to the nearest aircraft workshop. Thank goodness there was a knowledgeable and experienced LAME available. He checked the lines, then put a spanner on the fittings. Shock Horror!! These fittings which felt tight by hand were quite loose. After he tightened them the instruments worked just fine. The vac pump went up to the middle of the green, better than it had ever been since the autopilot work. Probably they had been leaking ever since that AP job.
We shouldn't jump to conclusions that previous pilots didn't report a fault on the maintenance release. There may well have been none to report. The vac lines or the fittings may have chosen a very bad time to start leaking. It would be very difficult to find any evidence of this after a fire. Even after 38 years of flying it amazes me how planes seem to be able to come up with new GOTCHAS.
Cheers, RA