The nothing that is a vacuum is a geometrical property, containing no rest mass, but it should contain 'gravity', being the metric defining it. It seems that a very small part of the universe can be defined as 'rest mass', with a atom containing 99.99% vacuum, if I remember it right. But you can exchange accelerations (and relative motion) for relativistic mass, and you can also exchange it into 'energy', creating a equivalence to mass, as rest mass. So 'energy' can become 'mass'. Without space in between all mass would become clumped together, and you can either think of that as a universe in where we still have distances existing, even without vacuum defining that property, or as something not making sense at all. What both a vacuum and proper mass has in common is its 'metric', 'gravity'. Assuming this to define geometric distances, also assuming that those distances (gravity) gets their definition from proper mass (energy) you then find gravity to blame for the universe we measure on. But that's my view.

As for not being certain what a vacuum is, I totally am of a same mind :) It's always seem to come down to some sort of symmetry in our universe, just think of how any force has a equal force acting in the opposite direction, which also explains how you can get a motion in space just by moving (displacing) your limbs relative some defined coordinate system. F = ma, where F is the net force acting on the object, m is the mass of the object and a is the acceleration of the object. Even though correct, meaning that we find it to be so testing, it doesn't explain 'why' it must be so, but it is a symmetry to me.