Hi
The VMO can be strengthened it can not be strengthened in isolation as once thought. I would go to a physio and have them check you out to make sure that the other things I mentioned, (weak hips allowing femoral internal rotation, excessive ankle pronation, or poor motor control causing incorrect biomechanics) are not occurring. If any these other things are present solely strengthening the quads is like swimming up hill. With as many dislocations as you have had I would suspect that there would be significant medial restraint laxity, and addressing all possible contributors will be required for an possible benefit. The next step would probably be a knee brace with a firm lateral "J" support to externally control the patella. Not sure on the success of those braces but worth a shot if the fixing the other factors fails.
Let me know if I can help with anything else
Kiwi DPT