If the listener has the volume too low and can't hear, the VibrAlert gives them an extra chance to reach their attention, making the marine VHF vibrate, which may be in a pocket.
It is a feature present only in some VHF of Cobra Marine.
VibrAlert
The VibrAlert allows you to vibrate the marine VHF to the other party, a bit like in common mobile phones with vibra-call.