Preheat the oven to 200˚F. Line a baking sheet with parchment paper
Core the apples using an apple corer, and thinly slice on a mandolin. On my small mandolin, which has three settings, I used the second setting. If the slices are too thin, it's difficult to get a full apple ring and they'll be more crispy. If too thick, they'll be very chewy and won't crisp easily.
Whisk the lemon juice, water, and vanilla together in a small bowl. Soak the apple slices, five or six slices at a time, for two minutes. Dry with paper towels and lay on the prepared baking sheet.
Place the curry powder in a small strainer and lightly dust the apple slices. A little curry goes a long way, so it's not necessary to dust both sides of the apple rings.
Bake for 45 minutes without opening the oven even once. Since you're baking them at a low temperature, it's easy to lose what heat you have. Turn the slice over and bake another 45 minutes, again without opening the oven. If they need a little more crisping, bake another 15 minutes.
For the crispiest texture, allow to cool before eating, but they're pretty fabulous slightly warm.