It's my experience that on-brand dealers ask a LOT for their used cars. Our Subaru dealer asks at-market prices for their non-Subaru vehicles, but WAY higher for their Subarus. A Forester that you'd see for $17-18K on a different lot will be $22-24K on the Subaru lot.
1. I guess they can get away with it. People looking for a Subaru go to a Subaru dealer thinking the dealer will have the really cherry models. Some truth to it, but I've found all my Subarus on other lots.
2. Want to show new car buyers that their car will hold it's value. I've heard it used in the new car showroom a few times: "We have a three year old car and it's barely less than this new one! Wow, you won't lose money buying a Subaru!"
3. Our particular Subaru dealers are really, really greedy. Their service rates are ridiculous, they 2x or 3x most parts prices, and their own service schedule would bankrupt a Saudi prince were it followed. So, I'm not surprised that they price their used vehicles very high too.
In the OP case, I'd just buy the one on the Mazda lot. Obviously the VW dealer is just looking to snag someone looking for a VW and not doing their homework. I doubt the one on the VW lot has any extra warranty. "Available for purchase."