Have there ever been accidents due to a vacuum failure that hasn't been noticed?
There have been many accidents because the vacuum system failed; AOPA has a good article that discusses some of them, and see this NTSB preliminary report for an example.

As for accidents caused because the pilot didn't notice, that's a more difficult question: if the pilot didn't survive then there's no definitive way to know what happened and what he experienced. It's always possible that the pilot was fully aware that the vacuum system failed, he just wasn't good enough at partial panel flying to keep the aircraft under control.