I've moved this from the VFX topic for discussion. I think it was actually designed to VFAX, the first requirement of that name prior immediately preceding the VFX.
Vought VFAX studies started from July 1965, their FX work about the same time. The final V-483-149 design from 1967 was VG, and when scaled up became the V-505, then V-507 VFX designs.
The other possibility is the design was aimed at the later VFAX requirement, but then the twin seat near-identical USAF model seen in American Secret Projects makes less sense - what requirement was it aimed at?