Are virtual photons and electron-positron pairs related in the polarized vacuum?

[TrickyDicky]

How are virtual photons and virtual electron-positrons pairs of the polarized vacuum related?
Are they interchangeable, do virtual pairs give rise to virtual photons? Or none of the above

 

[TrickyDicky]

I gather from the thread "When virtual becomes real" that there is a process called Delbruck scattering that is involved in the virtual photons-virtual pairs picture.
Could someone explain this a little? I think I might be mixing real photons with virtual ones here.