Yikes, sorry to hear you are having this side effect. I hope you have since switched medications.
Vytorin contains two ingredients, one of them is simvastatin which is a member of the Statin family (HMG-CoA inhibitors). They are known to cause muscle aches/pain/weakness in some patients. This is an undesirable side effect, obviously, and a patient should be switched to something else, even another Statin, or have the dose reduced if this side effect occurs. This indicates muscle breakdown, which can lead to kidney failure.
Also, it's important to not drink grapefruit juice or take certain medications (like some antifungals) while on a Statin because the levels of the Statin can be increased, thereby possibly increasing side effects.