We would not see it until (if) it hit us.

Observation cannot travel faster than the speed of light. No matter what it is you're using to observe: Photons (light and radiation), measuring gravity, heat, anything. No matter if the phenomenon's expansion were traveling at the speed of light, the changes to the universe being made as well as our ability to observe them are also traveling at the speed of light.

If the phenomenon were very far away, we would not be able to observe anything it was causing until its leading edge caught up to us. Then we would be destroyed at exactly the same time. This is because in your example it is expanding at exactly the same rate as the universal speed-of-light constraint allows us to receive any indication of its presence. Any evidence of, e.g. a far away star being destroyed would take X amount of time to reach us by its light no longer arriving. However, in that time the edge of the space-destroying phenomenon will also hit us, because it will also take exactly X amount of time to reach us, at the speed of light, from the point where the star was when it was destroyed. The distance is the same, the speed is the same. We would continue to receive light from that star in the meantime, as we already do. (The light from the stars you see in the sky now is already tens/hundreds/thousands/millions/etc. years old depending on the distance to the star in question.)

If the phenomenon were so far away that it is outside of our observable field of the universe, it will never reach us and we will never have any indication of its presence. That's what "observable universe" means. Anything can happen anywhere outside of the observable universe and it is objectively meaningless to us, because we will never ever be able to reach it, record it, have it influence us in any way. This is, however, predicated on the theory of the perpetually expanding universe being true (which it probably is).

If you want to actually see the stars in your sky winking out over the millennia, I suggest building your universal destruction bomb such that its shockwave travels at, say, half the speed of light or some other suitable fraction.

I think if it's outside the observable universe, then the answer is probably we would never observe it.

"Some say that this has already happened""

(anything outside of your lightcone can be ignored - there is no such thing as seeing it or knowing about it)

If the sphere of destruction is propagating at the speed of light, then any observable effect reaches you at the same time as the sphere itself. Either you don't observe it because you're far enough away to be safe, or you don't observe it because you're dead the instant it becomes observable.

Incidentally, you might be interested in looking up the idea of false vacuum decay - although if you tend to get anxious about end-of-the-world hypotheticals you might prefer to give it a miss.

Read somewhere that if the sun blew up we wouldn't see it before the gravity affected us.

Fun idea. You mean like the expansion of the universe is going 0.5 light speed at the edge of the void, so the spreading void is basically pulled back right? And then any photons that reached us from just before this void are traveling a tiny bit faster I guess? Being just outside the void?