For the chips: Place 1 rack at the top and 1 at the bottom position and preheat the oven to 300 degrees F. Line 2 large heavy baking sheets with parchment paper. Lightly coat the parchment with nonstick spray.
For the topping: In a small bowl, stir the sugar, cinnamon and salt until blended.
For the apple chips: Stand each apple upright. Cut 1/4 inch vertically off the right and left sides of each apple. Set the mandoline or v-slicer for cutting 1/16-inch-thick slices. Place 1 apple, cut-side down, on the slicer. Cut slices until the apple core is exposed. Turn the apple over and cut slices off the remaining side until the core is exposed. Arrange the apple slices in a single layer on one of the baking sheets. Repeat with the second apple.
Using your fingertips, sprinkle the apple slices generously with the topping. Bake until the edges curl and the chips are golden and feel crisp, 40 to 45 minutes. Pull the parchment with the apples onto the work surface and cool completely, 20 to 30 minutes (the chips will crisp more as they cool).
For the dip: In a medium bowl, stir the yogurt, maple syrup, vanilla and salt until blended. Cover and chill until ready to serve.
Serve the dip alongside the apple chips.