The rise to fame of disformal theories of gravity within the dark energy paradigm has been swift. Since their conception in the early 90s, it has been known they posses the capacity to vary the speed of light, and were utilized for this very function by John Moffat and others, as a viable alternative to inflation. But breaking Lorentz invariance this way opens the door to a wealth of new possibilities. In this talk, I will answer a long standing open question: can vacuum Cerenkov radiation occur in disformal theories? As we will see, this answer can in fact be yes, but with this yes comes a significant caveat – only if we violate the equivalence principle as well.