Simple Simon explained it the best. I agree 100% .. squats are FAR from the best exercise. The best exercise to isolate all the vastus is leg extensions done unulaterally. Try it. It ain't powerlifting, it's a smart man's game ..not a strong man's game
Why would you want to isolate if you can train them all at once?
Squats are very good for building quadriceps mass, BUT there is a point in Simple Simons reasoning.
It all comes down to biomechanics.
If you have short femurs, relatively long torso, you will squat quad dominant and yes squatting will be one of the best exercises for legs.
However, if you have a short torso, long legs, you will be a glute dominant lifter. Your lower back and glutes will do most of the work while doing squats. For these people there are better exercises than squats to build the legs (purely from a bb standpoint).