If you search my posts you will find a lot of info on how I have managed to build up an invisible VMO and very badly atrophied quads. I had bad treatment for a long time and last June my proper treatment began, my quads were so bad I could barely move my leg, my VMO was so bad my OS thought it was paralysed - he could not find it despite me straining to contract it as hard as I could.
I started with e-stim, 20 mins twice a day and then exercises that were specifically aimed at engaging the VMO, the VMO cannot be isolated, it is part of the quad set, but you can encourage it to work by making adjustments to exercises (turning your foot out by 10 degrees on an SLR for example). My whole exercise program is around here somewhere. My quads are now good size, my VMO is visible to the naked eye and on contraction (hard) its around 70% the size of the other side, however it is still very slow to contract and it seems I have to really strain hard to contract it hard enough to stabilise, my physio says my neuro recruitment is still slow.