VFIRE is supposed to be an interop group. To be used between more than one agency. Also it is in the LG part of the spectrum. Which is very limited already Not really appropriate for the Feds to use on their own small fires when they have gobs more spectrum available on the Fed side than LG/State will ever have.
VTAC is even worse to use, it is to be used for comms between different types of agencies such as law/fire.
I know it does not always get done like this, sometimes VFIRE are used on LG IA Tac, but nobody should try and make it worse. The V channels are very limited already.