Try doing this to see if you can feel your VMO working
Before performing advanced VMO exercises, specialists on the Sports Injury Clinic website suggest checking to see if the muscle group is functioning properly. Sit with your legs extended in front of you, with a towel placed under the back of one knee. Start with the knee slightly bent. Place your fingers on your VMO, and press down against the towel to extend your leg. If the muscle is activating correctly, you will feel it contract under your fingers. If you don't feel the VMO contract, it may not be strong enough for weight-bearing exercise. This sometimes happens after anterior cruciate ligament surgery. Continue practicing the towel pressing exercise until the muscle gains strength. Perform 20 repetitions every day.
The article went on to describe using a contraption called a Petersen Sled to strengthen the VMO:
The Poliquin Performance Center uses an unusual exercise called the Petersen sled drag for vastus medialis oblique rehabilitation and knee injury prevention. The sled consists of a metal platform, with a bar in the middle for attaching barbell plates, and a chain or heavy rope to pull the sled. After placing the weight plates on the sled, hold the rope or chain with your arms relaxed and extended. Keep your legs close together, with your feet slightly turned out. Drag the sled as you take small steps and walk backward for about 25 feet. Lift your heels from the floor as you walk. Perform four setshttp://www.dieselcrew.com/articles-pdf/ ... sledii.pdf
I've seen something similar to this sled in my gym. Do you have access to a gym that might have one?