When Vance doesn't believe he has the personnel, he goes into a soft zone, which is half of the problem. In the first game and most of the way through the second game, the Cardinals defense was playing man-up press, mostly with 1 safety over top, sometimes two. This freed up the linebackers as well, as they had man coverage. Remember all of those replays of Kittle getting shut down over the middle, or constantly with a man trailing him? We just about took the TE out of that game by playing a consistent man-to-man defense.
Compare/contrast that with the Lions game, and with multiple safeties going down and not enough of a pass rush to compensate, VJ goes into soft zone. Keep the ball in front of you. His zones are too soft and this leads to death by 1000 papercuts as Stafford was able to cut up the middle of the field against us. You can tell VJ is not confident in his players when they play a preponderance of soft zone, whether it's quarters or 3 deep, and he's feeling like he has the players he needs when he plays man. We'll see how much of each plays today, but I'm betting it's primarily zone and the opposing QB will have a lot of 7-11 yard completions.
I hope I'm wrong.