vastus medialis exercises?

I agree with Daniel, doing step ups, lunges, wall squats etc will engage a lot of the vastus lateralis and you do not want that to take over. The leg extension, but only that last 30-40% of extension is the movement I find really beneficial. But be conscious of tempo the eccentric phase (the way down) needs to be slow to cause micro-trauma in the muscle fibers and promote muscle growth. I go for a 2-2-1 tempo. 2seconds hold at the top, 2seconds down, 1second up.

Hope this helps