"Collapse of the vacuum" is a high energy physics thing, referring to what happens as the universe cools - if the forces we can measure at "normal" temperatures aren't fundamental, then different force strengths could be possible, if the fundamental forces are arranged differently. This leads to the idea of the false vacuum and related stuff.

Anyhow.

If I understand decoherence right, then quantum events which can't be differentiated from each other get summed together into the same blob of amplitude.

Nope. The second part of this sentence is true (as an empirical fact we've learned about the world, not just something that happens automatically), but doesn't relate to decoherence. Instead, it's the quantum-mechanical definition of identical particles.

the extreme evenness of a vacuum might have to do more with the overall blob of amplitude of the vacuum being smeared out among all the equally-likely vacuum fluctuations

Well, if we measure the state of the vacuum in order to know that it's even, then it's not very un-differentiable now is it?