The Vikings paid AP $12M in 2016, all guaranteed. The Vikings are due to pay AP a whopping $18M in 2017, none of it is guaranteed.
First of all, $18M is an absurd salary for any running back, even AP. That would make him paid at a rate of one of the top-20 players in the league. Even if he had his best season ever in 2017, it's arguably still not worth it to pay any running back that kind of money in this day and age. To put this in context, the next highest paid running back is currently Jamaal Charles, who is getting $9M. Shady is getting $8M. Ezekiel Elliott and DeMarco Murray are each getting about $6M. Do we really think past-his-prime AP is worth 2-3 times as much as these guys? No way!
Now arguably, the contract was set up with the expectation that the Vikings would not pay that salary unless AP ascended to super human status and was balling out like crazy. It was also probably set up to ensure that the Vikings probably can't trade him away to somewhere AP doesn't want to go (unless the trading team really, really, really liked him) -- the contract is so much the Vikings would sort of be forced to cut him if they didn't want to cough up the money. The reasonable expectation then, probably by both parties, was that the Vikings and AP would at least attempt to renegotiate in 2017. Or just part ways.
This contract will likely not exist in 2017, so the real question is do we think the Vikings and AP can renegotiate. In the Vikings' favor, they have a coach AP likes and respects and they have a defense that could conceivably take them on a deep playoff run. However, going against the Vikings, they are probably not yet Super Bowl contenders (AP wants that ring), they lack an offensive line (AP knows he won't do well if they don't fix it), AP probably still hates and hasn't forgiven the FO for what he sees as stabbing him in the back (child abuse scandal), and AP is probably going to want to get paid a lot more money than anyone will think he is worth (a realistic Vikings offer might be interpreted by AP as a slap in the face, and I could see him walking way in a tantrum -- the last contract dispute between AP and the Vikings was protracted in ugly -- I could see AP wanting another fierce fight and I don't know if the FO would bother to put the energy into it anymore). Another factor: The Vikings offensive line failure this season is so significant that focusing energy in the off-season fixing that is likely to be a far more important priority to the Vikings than trying to spend resources to keep AP. After all, if the offensive line is this bad next year, it wouldn't matter if AP was in his prime or not, he wouldn't be running very far no matter what.
TL;DR: Vikings will likely have much more important uses for the kind of money AP is likely to demand. There is probably little chance for a deal.