I would argue that a vertical forearm grip is not really legally/financially practical for the T/C shooter.
Having vertical grips on rifles doesn't invoke special ATF rules, but putting a vertical grip on a handgun puts it into the "an other weapon" category since now it is no longer designed to be fired with one hand.
You can make one, but it of course requires forms, $200 and ATF approval to get started or only $5 to transfer one from an authorized manufacturer.
http://www.ihmsa.org/BATFE.htmIs it reasonable, especially for a single shot T/C? Since when did expect our government bureaucracies to be reasonable...