As far I know, if such an event would happen, we couldn't see it coming:
A hypothetical vacuum metastability event would be theoretically possible if our universe were part of a metastable (false) vacuum in the first place, an issue that was highly theoretical and far from resolved in 1982. A false vacuum is one that appears stable, and is stable within certain limits and conditions, but is capable of being disrupted and entering a different state which is more stable. If this were the case, a bubble of lower-energy vacuum could come to exist by chance or otherwise in our universe, and catalyze the conversion of our universe to a lower energy state in a volume expanding at nearly the speed of light, destroying all of the observable universe without forewarning.
What about one or more that might have happened in the past, and engulfed the visible universe?