The vmo can definitely be an issue, especially dependent on the training background. Single leg squats can help if they dont aggravate the injury due to the instability, forcing the vmo to work harder and contribute more, even with reduced rom. However, most of the time the vmo issues tend to show up in guys that have been deadlifting/box squatting/squatting high or parallel in my experience and not nearly as frequently in athletes that have been full squatting for the majority of their training.
Its fairly rare to see someone who has been squatting full have much of an issue with the vmo, and much more common for them to have a hamstring issue, especially at the knee end of the spectrum. High rep hamstring curls done with a high frequency are unbelievably effective for this, giving both a tonic blood flow effect to the knee and prehab as well. Doing things like rdl's and deadlifts dont work as well in this case as it tends to hit more at the hip joint rather than the knee end.
lbss brings up a good point too, be careful taking random advice on injuries from someone on the internet. Its always good to hear different peoples experiences with similar situations, but there are a lot of people who go around reading different articles/forums online, then state what they read as fact, without any prior experience or knowledge of what they are speaking about.
As far as dreyth in this case, if you dont remember a specific time and place you felt the knee "give" or "shift", or any type of acute pain/injury, the odds are in favor of a tendonitis issue but thats definitely not fool proof. The way you squat would definitely favor the vmo and overall quad development if I was making an educated guess at it, and you may very likely need to do some remedial hamstring work at the KNEE END. If money is an issue then trying that while making sure mobility and soft tissue work is up to par first, if not then getting a professional to look at it is always a good idea.
