There is some Alpine-Vansco info here
It is easy to see if the rear unit is powered- there is a plug giving 12V to it w/~#10 wire. Untwist that plug & check for 12V. If there is no 12V, you need to start tracing from there. You may have a fuse or circuit breaker blown/tripping in the ignition circuits. There is a "mega" fuse, 250A on the back wall of batt compartment, and another in the rear-most compartment above the muffler; you can check continuity across the studs for those.
There is a twisted pair that sends "network" signals between front & rear VMMs so that an input on the front can translate to an output on the rear or vice versa. However, with your limited description of the non-Vansco failures, I'm guessing it is in general coach/engine wiring, not a Vansco failure per se.
Let us know what else you find.