I recently caught my VT biting himself right before all his fins were fully healed (dorsal, anal and ventral). he would bend his body against anything to catch his fin and chomp it right off. I didn't know what was wrong because nothing had changed since he started doing it. His water conditions are perfect, so I was really confused. I think it was a combination of boredom and swimming space. I searched the forums and did the following:
1) I removed some plants and arranged some in such a way that would shield him from view (some say their fish prefer heavily planted tanks, but it seemed to stress him out bc he is quite a swimmer). Try playing with combinations of heavily planted areas and open water.
2) I made sure there was a dark tunnel or object to swim in and out of. I bought a small black castle that I tipped over so he could hide. My betta seems to love swimming through cracks and tight spaces, so I arranged the castle against his sponge bob pineapple house and he now swims through a series of entrances and exits. Just be make sure there is enough space for your fish to get in and out safely.
3) I switched to feeding him flakes, i noticed that he would be preoccupied for quite some time looking for flakes that had fallen to the bottom. Unfortunately this means I'll be cleaning his tank twice/week instead of once.
4) I reduced the flow of his tank, so that the surface water was fairly still and made sure there was no current pushing him around his tank.
Since then, he has been healing up slowly. I'm adding some aquarium salt with every water change and a drop or two of medication to avoid fin rot. He also seems to be enjoying his new surroundings swimming all over his tank.
Hope this helps,