Wolfsburg has more spending power than Juve, thats true, their usual revenue is low but they still manage to buy players like diego, luiz gustavo and maybe de bruyne, actually wolfsburg has more spending power than bayern.
VW is not only the sponsor, the clubs belongs about 90 % to VW, did you really think that a small club like wolfsburg can pay that amount of money for this kind of players? If VW wants they can pump an huge amount of money into wolfsburg, but the difference between VW for example with Etihad from manchester city is that VW is a serious company, with well-grounded structure, they try to invest sensible, but as I said they also can go full retard and spend about 100 Mio. if they want.
they try to build something really big in wolfsburg, they want to play in 2-3 years champions league and also in near future to be a big gun in germany aswell, a title contender.
VW uses football as a platform for their own business, especially for european market and also international market.