Vacuum decay (or sudden phase change) is not related directly to accelerated expansion of the universe (at least to our current knowledge) which is being driven by dark energy.

In one situation, expansion simply keeps on going, giving us the Big Freeze.

In other situations, dark energy at some point becomes attractive and the universe starts to contract and ends up in a Big Bounce.

Vacuum decay is different. It is a sudden shift of the lowest level of energy of the Higgs field. We wouldn't see it coming at all.