The most accepted theory about the beginning of the universe is the classical inflationary big bang theory. It states that the universe emerged out of a vacuum fluctuation and that space-time itself was created along the bang in the fluctuation. But we say that these vacuum fluctuations take place in the fabric of space-time itself, then this should mean that the fabric of space-time was always there even before the big bang. Am I misunderstanding something?
There are different theories for quantum gravity and quantum cosmology. None have been accepted at this point. It includes string theory models, models that use the holographic principle, a loop quantum gravity theory and others. See a review of quantum cosmology at http://www.damtp.cam.ac.uk/research/gr/public/qg_home.html
Some of the theories posit something that is there before spacetime emerges at a more macroscopic level. The degrees of freedom or descriptions of those pre spacetime construct also vary. No sense my summarizing it here, there's review articles like the one I referenced, and others as well. It is an unresolved question.
Inflation is often taken to start at the Planck time as that is the earliest we can say non-quantum gravity physics starts to apply. A preinflationary epoch is proposed in "quantum cosmology" where one tries to quantize the Hubble equation. But that is some what separate from standard inflationary theory. In general anything before inflation does not leave an observational signature as inflation has attractor solutions that effectively erase any sensitivity to prior epcohs.