Some studies have found that using an estim machine can help the VMO activate. Also terminal knee extensions with a theraband are helpful (even if they look like they are ridiculous).
If you aren't sure if it's activating at all, sit down with a towel under your knee. See if you can tighten through the quad to lift your foot from the floor. If you put your fingers on the VMO, you should feel it contract. Work on holding that.