I don't know what case(s) would exist in TN, but they would likely not be the same in WI (where you are apparently from). I reside in IL, and a quick search on Google revealed that there are two cases under the Illinois sales tax (on vehicles, they like to call it "road use tax") where one might get a pass on the tax, other than the usual (the amount of a trade-in deducted from the sale price of the vehicle). Neither of these cases apply to the buyback, so I expect we will pay the full 6.25% when we buy a new vehicle, just as we did when we bought the Passat we have now (there was no trade).
So, I think the point is that the issue of sales tax must be researched on a state-by-state basis, and so far I think the only state that someone said has a loophole that allows the buyback to slide through is VT.
And strictly as commentary, I can't see why any state would feel any obligation or motivation to change the rules or pass a one-time statutory exemption for the Dieselgate buyback. I'm just not coming up with an argument that I feel would wash with the IL Dept. of Revenue. And saying "Please let me out of paying this tax" is certainly not going to walk.