That sounds like a really bad idea tbh. It was such a disaster last time, and Becker has one of the games made most useless by current court speeds and balls- might help his overhead though.
Djokovic's problems are mental. From the Berdych loss onwards, he was choking matches left and right- he wasn't really decisively outplayed in a single match I can think of other than his no-show at Wimbledon
Becoming a better all round player will never hurt anyone.
And Becker wasn't an out and out serve and volley player like some seem to think he was on here.
I also seriously doubt that Becker is taking over as Djokovic's main coach. Perhaps he will be at the tournaments Vajda is not present at, but that still doesn't mean much.