Research would tend to indicate that preferential strengthening of the vastus medialis (VM) is not possible (Smith et al 2009).

Clinical observation and practice would indicate that it is possible.
The significant wasting observed in the VM in preference to vastus lateralis (VL) in knee joint dysfunction/injury would indicate some sort of differential activation mechanism.
Differential neural innervation between the different segments has not been demonstrated to explain the clinical observation.
However, Franke et al 2014 concluded that higher increases in muscle mass and muscle activity observed in the medial portion of the quadriceps femoris suggest that VM strengthening was greater than VL strengthening with eccentric training performed on the isokinetic dynamometer.
Clinically when an emphasis is placed on maintaining a normal anatomical alignment, and subtle weight shift onto the medial aspect of the heel, then a more effective response is obtained in the VM and GMax by the client during exercises such as the wall squat, arabesque squat or split lunge.
This in turn has been shown to decrease the clients pain response in their knee.
If the non-anatomical alignment is adopted eg a dynamic knee valgus(DKV) then the VL can be more activated and have a strain response which in turn can increase the pain response in the knee.
I use the Smartherapist app to reinforce the correct exercise performance and assist in maintaining the subtle weight shifts and positioning required to get a more specific muscle response using the video feature.
The reminder feature assists with compliance and ensures repetitive and regular performance of the correct exercise technique to reinforce and learn the new motor program.