There seems to be a lot of confusion about virtual particles with regard to ex nihilo.
Specifically, people seem to think that space is exactly equal to nothingness.
Just some quick points.
Space is expanding. So I guess it gains somethingness just long enough to expand and then becomes nothing again?
Warping space causes light rays to bend. They're travelling though nothing, which by definition can have no properties, but bending the nothing causes an interaction. Right.
Finally, empty space has an energy density. That's pretty impressive for a nothing.
So I guess the ex nihilo problem isn't virtual particles, it's that there's an entire ex nihilo energy density. (Which I believe is currently linked to dark energy.)
In reality, pure nothingness is very different from the vacuum. The vacuum is simply a backdrop for particles.
To test this statement, consider the first moments of the universe. By the Big Bang theory, it started as a singularity. Everything, including space itself, was packed into a single point. (Something that is normally impossible, but the Big Bang really did come from nothing.)
Shortly after, the universe is a finite size, and a very small size at that. What happens when the particles hit the edge?
If space is really nothingness, of course the phrase "space was packed into a single point" is meaningless, and they just kind of keep going. Whoosh. (This contradicts the idea that the universe is about 44 billion light years across, of course, since nothingness has no definite size.)