Don't fall for the obsession with isolating the VMO (vastus medialis obliquus) muscle as a way to strengthen the knee. The VMO is one of the quadriceps muscles–the one that attaches to the inside of the knee–and it is often underdeveloped compared to the other quad muscles. This has lead some to suggest strengthening it through isloation exercises. Unfortunately, it's really not possible to isolate the VMO. Instead, you should focus on multiple joint movements that will lead to greater strength in all the major muscle groups in your leg.
If you are under a doctor's or physical therapist's care and he or she has designed a regime of isolation exercises, such as terminal knee extensions (TKEs), follow those instructions. Otherwise, incorporate such lifts as squats and deadlifts into your exercise plan. Start with light resistance, focus on form and build strength carefully.
Although the big moves mentioned above are important, there is also a great exercise that specifically strengthens the stabilizing muscles around the knee: the Peterson step-up. The following video describes the mechanics:
If you would like to have a comprehensive knee health program, specifically designed for preventing or recovering from a torn ACL, be sure to check out Understanding and Preventing Noncontact ACL Injuries.