It's been made official recently.
Honestly, I'm kind of surprised by the appointment given how his Bremen teams have collapsed in the past, but maybe this Wolfsburg team's better suited to his system. He demands a good amount of effort from his players, and this team seems to be able to cover more ground than his Bremen teams.
In his first season he stepped in for Nouri early and steadied the ship, in his second season they placed 8th with some genuinely encouraging football. In his third season is when arguably when the collapse occured: glass half full is stating that they sold Kruse, Füllkrug (their designated goal getter) got injured very early into the season and Rashica just checked out halfway through, leaving them with a terrible lack of individual quality in attack, glass half empty is stating that the team wasn't just blunt upfront, but genuinely unstable the longer the season went on, while he also abandoned his positive football principles - unlike for example Nagelsmann, who turned Hoffenheim around in style. In his final season the management had to sell of Klaassen and let go players such as Bartels, Sahin and Pizarro, who I imagine were important to the dressing room at the very least - even though the team only stayed in by the smallest of margins the year before. I wouldn't necessarily hold the relegation against him, the club as a whole was simply a mess.
He's very charismatic, the big question is whether he can play and improve the football of his first two seasons at a better club, how much of the mess of his last two seasons were down to Bremen's problems as a club and how much were his own mistakes. It's a surprising appointment in the sense that I probably would have expected his next club to be more around lower midtable, but on the other hand Wolfsburg have one of the league's worst ever coaches as their caretaker and so early into the season they probably don't have a lot of candidates to choose from.