Hi, Nernico,
Welcome to PF!
I think you've misunderstood something about the quantum vacuum. You seem to be visualizing it as a kind of empty space that predated the big bang. That isn't right. There is no viable cosmological model in which empty space predated the big bang. "Quantum vacuum" is just an ordinary vacuum. The "quantum" part is just a way of emphasizing that an ordinary vacuum, such as the one that exists between the atoms of air in this room, isn't really empty: it contains particles that pop in and out of existence.
First off, we don't have a theory of quantum gravity, so there is no way that we can confidently answer questions like this using quantum mechanics. The best we can really do based on well-tested theories is to answer this kind of question using general relativity, which is classical, not quantum-mechanical.
So in terms of GR, the picture we have is that time and space exist only for t>0. For any t>0, there is an earlier time, so we can always give cause and effect explanations. Conditions at any given time were caused by conditions at an earlier time. GR can't answer questions like why there is something rather than nothing. It can, however, say that there was something rather than nothing at time t because there was something rather than nothing at an earlier time. In GR's model, there is never any creation of matter out of nothing (in technical terms, mass-energy is locally conserved). Matter exists at all t>0, and there was never a universe full of empty space.
GR's description of cosmology is expected to be wrong due to quantum-mechanical effects at times earlier than the Planck time, which was about t=10-43 s. There are speculative attempts to construct a quantum-mechanical theory of gravity that would fix these problems. Marcus works in that field, and he will probably chime in with a better description of that than I can give.
Hey, Marcus, this question clearly deserves a FAQ entry -- want to write one? If so, feel free to use my material above if you find it useful.
-Ben
