Virtual matter can not give off photons - it would violate conservation of momentum (unless it was in pairs, but that would not work with Cherenkov radiation).
There seems to be one paper about Cherenkov radiation in a vacuum [1]. But I have only read the abstract. So you are probably right, that there's no Cherenkov radiation in a vacuum. Or does anyone has any better knowledge?
[1] "Cherenkov radiation in vacuum and plasma-filled microwave sources in the absence of guiding magnetic fields" by GS Nusinovich
