I think pinning the defensive frustrations of the last two seasons on BVG is a little short sighted, and over-simplified.
The biggest knock is that BVG's defense needs the right kind of players, skill-wise, with very aggressive tendencies, and extreme ball-smarts (for the college level, not so much for the average NFL player.)
I think BVG is going to make less modification to simplify than many assume, and the defense is going to produce far better results. It starts with a smarter backend, no matter who starts, it will be much smarter. (And oh, by the way, one of our starting corners was playing with a leg fracture last year. And one of our safeties tested incredibly slow for a system that relies in man coverage, and both showed no football smarts, though I love them both.)
The front of the defense is going to be much 'skronger' this year, which will have an 'incredible impact!