After becoming extremely bored while studying for an Afrikaans exam, I started thinking about virtual particles. So, can light (photons) interact with virtual particles (even though they only exist for a very short time)? And can these virtual particles "slow down" light in the same way as atoms delay light when travelling through a medium?
Yes, light can interact with "virtual particles". It can also interact with itself via virtual particle interactions (see Delbruck Scattering), although I believe direct observation of this effect is currently outside of our experimental capability.
Edit: Just realised I didn't address the second part. When a photon propagates, the propagation receives contributions from its splitting into an electron-positron pair which recombine etc. These processes contribute corrections to the photon propagator, but they do so in such a way that the propagator pole remains in the same place, which in turn means that the propagation speed is unaffected.