I don't have the Galaxy S III, but I did have the Galaxy S II and it had the same issue. Also, I didn't like how I couldn't move the phone to silent (mute, not vibrate) with the volume rocker and it seemed like no setting would give it to me easily, unless maybe I went into the Sound Settings, which is a bit too much just to mute your phone. I always keep my phone either on vibrate or mute... I have read the Galaxy S III manual for T-Mobile and it looks like the mute option is easier, so I will likely give them a try again.
To answer your question, I found this out on the Galaxy S II. Settings --> Sound --> Vibration intensity. What I didn't like is that it increased my Haptic feedback on my text messaging and I didn't want that. I just wanted to feel the phone when I was getting a call or a text. Hope this helps.