Comparing JVB and Stanzi is difficult because they have different strengths and weaknesses.
JVB is already a better shotgun and straight drop passer. As far as standing tall and throwing a football to a spot, he's better than Stanzi. He is improving in PA passing. This has benefited the running game and the improved running game has benefited PA passing. The two go hand in hand so both were struggling at the beginning of the season and both are improving now.
Stanzi was a better PA passer and I don't think JVB will ever even get close to being as good as Stanzi at throwing on the run. That's why roll-outs, bootlegs, and PA passing was so good for Stanzi. He was really, really good at these passes, which goes really well with a good running game. He was an outstanding compliment to the Iowa grind-it-out offense.
JVB needs to keep getting better at PA. To be honest, I don't have much hope that he'll ever be great at thowing on the move. I hope he proves me wrong. Right now I would call him terrible at those throws. If he can get to serviceable, that would be great.