A nice, referenced review on a often hotly debated topic. 

“Can you emphasize vastus medialis activation in squats?Reading time:By Chris Beardsley, S&C Research columnistSome strength coaches have suggested that squats with either a narrow stance or with elevated heels can preferentially increase vastus medialis activation. They argue that training the vastus medialis is essential, because they believe it is more important than the other quadriceps at the bottom of the squat.But can we actually emphasize vastus medialis activation in the squat by changing stance width or adding a heel lift?And if we can, is the vastus medialis really more important at the bottom of the squat than the other quadriceps, anyway? Let’s take a look!”

link to full text: http://www.strengthandconditioningresearch.com/perspectives/vastus-medialis-squats/