Nigel, personally I'd prefer a dedicated speaker in the belief that there's one less switch, etc. that can fail or cause a problem cross-connecting the VHF and stereo. No big deal really to find an inexpensive speaker with a good grill
and dedicate it to the job.
You might also check your stereo, some of them have a "cell phone
mute" wire that mutes the stereo when a cell phone
(or in your case, VHF) audio output is detected. And some of those allow the "cell phone" audio to be played over the stereo's speakers, automatically.