In the standing med ball toss I would try to push my hips back further, like trying to touch a wall with my butt backwards. So basically, more of a hip hinge/hip action. In your case it seems like the chest just drops down forward and the hips don't get too much displacement/ROM.
By the way - what happens when you wear the weight vest and jump around with it? In my case, the last time I used it, my tendency was to become more quad dominant. I think it's interesting because it tends to show you what your dominance is, if you will, once you get loaded with additional weight. Hard to determine what this means, considering the distribution of the added mass, but still, interesting to note.
