It would have been more interesting (and more valuable) if they had snagged
Hendrix for an endorsement, but although he often used a Vox, he became
associated with the Dunlop Crybaby and the Prunes ad may well have been
Vox's response.
Eventually, Crybaby made "Jini Hendrix" models (at least two, the JH Signature
and the combo Fuzz/Wah but not as early as '68. As far as I know Jimi never
used the combo pedal, prefering his old Dallas Arbiter "Fuzz Face" - later
reproduced by Dunlop).
On many of his studio recordings Jimi used the Vox Clyde McCoy model Wah,
but it wasn't well known that he switched off. He was also associated with
Marshall almost exclusively, but I've seen photos of him using HiWatts, Kustoms
and the night I saw him (opening for The Monkees) he used Fender Twins and a
stack of Vox Cabinets.