How so. How do you/we know this? I can tell you it's not by taking our observations backwards.
You claim a vacuum existed before the singularity? Your visualising a tiny fireball in a vacuum exploding and expanding? The vacuum you refer to is a rich structure of fluctuating energy, so sub atomic particles don't just appear from nothing their a result of fluctuating energy in the vacuum, thus constituting to matter. Time, space and matter were all condensed in to a singularity. Upon expansion time, space (vacuum) and matter came into existence.
What empirical data and observations combined with physics etc show that the universe began, this is fact, the theories which refer to possible physical causes which created the singularity have no basis of proof where as the beginning of the universe is established fact.