The night before this happened, I lost power assist on my brakes driving home in the neighborhood. I've been chasing an oil leak, so I used the vac. loss as an excuse to pull it out, look at it and replace the o-ring at the base of the vac. pump. I ran it for a few minutes after replacing the o-ring and called it a day. The next morning, I started it to drive off, and the vac assist was back. "ok, I guess it fixed itself" (it was a rotary vane pump, I didn't suspect trouble). I drove down the block and suddenly a shrieking started, and the oil light turned on about 3 seconds after the noise. I had it shut down within 3 seconds after the light. I could smell a little burning rubber after I stopped.
Sure enough, the back of the timing belt had the impression of the freshly polished IM shaft pulley in the rubber, but the shaft didn't break! I am careful not to over-tighten the belt, I use the "45 degree twist" rule.
I swapped out the seized vacuum pump for a spare rotary vane pump I had on my core engine, and all is well. I had Schaffer's molybdenum EP additive in the engine, which may have helped. It still runs great, no new or ominous noises, so I think I'm ok. Still trying to find the leak, (think it might be a galley plug) but I'm thankful that I dodged the bullet on what could have easily been a catastrophic failure. Thought you guys would be interested to know.