I am goingto jump in, but I am not as knowledgeable about dressage, and certainly not about eventing, as many ohters.
I have seen you riding Vinnie in some other threads you posted and I know how beautifully and lightly you ride and how lightly he goes.
Never having held his reins in my hands, I am not sure if he is genuine to the bit or if he comes behind it (as many WP horses do) but I think he's pretty genuiine from what I saw.
I don't know what to say about a driving seat. But can I say something about the second part of your "two, two threads in one!" thread;
I would start working with V and work on getting him to accept more constant and steadying contact. To do this I would work on stretching and contracting him longitudinally. I would get him to the bit, then advance your hands and encourage him to follow the bit down and forward, while not losing forward marching walk rythm. YOu follow him down and you try not to lean too far forward, then pause for a few steps and use your seat to really push him "through" the gate that you have from your elbows to his mouth. Almost visualize it as if you are holding a jumprope out in front of him and you want him to step over the rope and through it , with energy and thrust from behind. Your seat will be doing the most work her because if you put your leg on too much, he'll pop his head up and you'll lose the throughness coming up from hind end , through the back, and over the "jumprope".
Get just one or two good steps and then think of keeping that same energy but contracting it. You will raise your hands and bring them back to your core, but you do it in rythm with his stepping, taking each side of his body as he advances that side with the push of the hind leg. You will feel the horse step into the bridle alternately, each side of the body that stretchs out when the horse pushes off on that side causes the horses head to ocilate a bit from side to side. When you feel the horse step into the right rein, you "take" a tiny bit more on that side, asking the horse to shorten his frame and collect. Step into the left, you take a tiny bit more there. By doing this , the horse shortens into a more collected walk, and you must drive forward with your seat to core. Get a couple of good steps and then back down again. After once or twice in each direction, give a total walk on a loose rein reward.
Longitudinal stretching is really valuable training excersize.