I like to work the VMO in prone. When you fully dorsiflex the foot there is a natural knee flexion. Have them straighten out their knee. It will also stretch the hamstring. For older men, with tight hamstrings and hip flexors, I find this exercise very useful. You can always drape a leg weight over the back of the knee for more resistance.
Keep in mind that all 4 of the quadriceps muscles insert into the quad tendon superior to the knee. So, whenever knee extension is involved all 4 of the quad muscles contribute to the movement. Because the rectus femoris also crosses the hip joint, it is also a mover for hip flexion.
To fully engage the medial vastus, be sure that the exercise overload take the knee to full extension. Karin refers to it as “terminal knee extension.”
One of the best exercises I know is performing the last 45 degrees of knee extension on a seated knee extension machine for 10-12 RM.
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