the question of what was "before" the Big Bang is a nonsensical question (scientifically speaking).

Agreed. Yet many serious scientists acknowledge the possibility of a big-bouncing universe. I merely state that any such suggestion is non-scientific, in that it's not subject to observational verification.

en.wikipedia.org - Big Crunch

There was some talk about the origin being a large vacuum fluctuation, though I would then ask "vacuum fluctuation in what" ... if the universe didn't exist then what was the fluctuation in?

Well said. While the notion of a vacuum may be macroscopically relevant in representing organically evolving perception; it's irrelevant in a quantum realm interfacing with infinite potential. The vacuum is anthropically perceived — associated quarks collectively experiencing existence.

That's what ephemerally existing quarks do. They arguably derive from original singularity. Individual quarks experience atemporal realization by virtue of relations eventuating within cosmic experience.

vacuum fluctuations produce symmetry; equal amounts of matter and anti-matter. So why would there be an imbalance/asymmetry if the universe began that way?

I've long wondered about that, and haven't seen anything that resolves the issue. Likely much more is transpiring in that research domain.

My guess is that the becoming of collective experience disrupts the matter/antimatter balance. Existence may come at a cost. While we here-now observe a minuscule imbalance between matter and antimatter, I suspect there's much more to it.

I tend to discount any human-based interpretations

While that precaution is certainly warranted, taken to the extreme it leaves us with nothing, as any interpretation we might be privy to is human based. We're ever challenged to do our best with what we've got.

Your thoughts?

Tom