How the F sound looks and how it is made:
The inner lower lip contracting as the upper teeth press into them to produce constriction produces this sound. The tongue placement can vary depending on the word, and the lower mandible is elevated. The feeling with this sound is that of the teeth pressing into the lower lip, as if biting.
How the V sound looks and how it is made:
The tongue is low, and the jaw (lower mandible) is elevated. The sound produces vibration in the vocal cords. The feeling with this sound is that the teeth are biting into the lower lip in such a way, but the sound produces a vibration at the same time, like a buzzing on the lip rather than just a bite.
Lipreading.org