The reason for this happening is that the VV device is emitting RF interference and the radio is vulnerable to RF interference.
The actual mechanisms vary but one example is that the VV device is chopping a voltage at a frequency the same as the radio's IF stage; or a strong harmonic is emitted at one level above, causing the interference. Once electronic circuits used in APVs reach 20,000 hertz (cycles per second) or above, the likelihood of this happening increases substantially. You can spend a lot of money protecting radios against this or reducing the interference transmitted by the offending device; but in this case it doesn't seem worth it since the range of the interference from the APV will be fairly low, as the power output is limited and the antenna distributing the interference is inefficient.
It would be interesting to experiment to see if the metal body of the APV is the antenna, or alternatively partially shielding an internal antenna, but there is probably no benefit to knowing unless you are driven crazy by regular interference of a four-second period while listening to a Mozart symphony while driving. If so, get a wire and touch the metal body of the APV to bare metal on the car chassis or other available earth, and see how it affects the interference...
p.s. Don't do this if you have a faulty design with a positive casing, or a basic APV with the battery inserted the wrong way round. A case that is the positive pole is a fault although many users (and even 'designers') won't realise this.
