[Tamara]

LukeA is correct: adding a VFG to the Kel-Tec would be manufacturing an "Any Other Weapon"

See the FAQ at titleii.com:

ATF has made the decision that a handgun (but not a machine gun,
since a machine gun is not also an AOW) with more than one hand
grip at an angle tot eh bore is an AOW. This is based on the gun
a) being concealable on the person, and b) not meeting the
definition of a "pistol" in the regulations promulgated under the
NFA, since they say a pistol has a single grip at an angle to the
bore. However, at least one federal magistrate has decided that if
the grip is added later, the gun is not "originally designed" to be
fired by holding in more than one grip, and thus putting a second
grip on a pistol does not make it an AOW. ATF does not regard the
decision as binding. The case is U.S. v. Davis, Crim No. 8:93-106
(D.S.C. 1993) (Report of Magistrate, June 21, 1993). The
prosecution was dismissed at the request of the Government before
any review of that determination by the trial judge.