Basically, if a vacuum metastability event happens, we'll literally never see it coming.
The only time you really could would be if there was some quantum-entangled transmission that suddenly stopped because the other side was hit by the event.
More fiction (sorry), but there's a book I read
1 which involved a galaxy full of humans (mostly trans-humans, in various ways) recovering from some kind of metapocalypse. There were many different kinds of transhumanism, but many of these inhabitants were people who with multiple bodies spread across the galaxy but possessed a single (multiplexed) consciousness connected by speed-of-light mental communication. Who somehow managed to deal with centuries or even millennia of 'lag' as one 'part' of their selves experiences and thinks thoughts that will not be known by another part of themselves until the transmission of this bridges the gap.
And then this "metapocalypse" happens happens. Whatever it is. I think spreading from the centre of the galaxy. At the same universal speed-limit. Whatever else it does, it certainly disrupts these individuals. All of a sudden each instance of the individual are disconnected from (have received the last information from) their counterpart instances closer to the original problem. But with no warning because the warning would have had to have come along following the disturbance, and thus cannot. Because the prior-effected counterparts (like the current counterpart now is) are now isolated and likewise have no way of sending any warning onwards to the yet-to-be-affected entities in this individual, blissfully unaware of the disastrous wavefront of the whatever-it-is that is yet to get to them.
Cue a form of galactic anarchy.
1 Got it from a Remaindered book store, so while the book itself ends inconclusively with the promise of a sequel, I don't think one will exist. I must dig it up and do a search, though, just to see.