Make the caramel: Combine the granulated sugar, 1/3 cup water and the salt in a large skillet over medium heat, stirring just until the sugar dissolves. Cook, gently swirling the skillet occasionally but not stirring, until amber, 8 to 10 minutes. Brush any sugar crystals off the side of the skillet with a wet pastry brush. Remove from the heat and whisk in 3 tablespoons butter and the vinegar (the mixture will bubble).
Add the apples and cook, stirring occasionally, until soft, about 12 minutes. Strain the apples and transfer to a bowl; set the caramel aside. Toss 1 tablespoon gingersnap crumbs and the cinnamon with the apples; let cool completely.
Melt the remaining 5 tablespoons butter in a small saucepan over medium-low heat. Let cool, then skim off the foam and discard. Spoon the melted butter into a bowl, leaving any solids in the pan.
Position a rack in the lower third of the oven and preheat to 375 degrees F. Line a baking sheet with parchment paper. Place a sheet of phyllo on a clean kitchen towel or piece of parchment with a long side in front of you. (Keep the remaining phyllo covered with a damp towel.) Lightly brush the phyllo with some of the melted butter and sprinkle with granulated sugar. Layer another sheet of phyllo on top; brush with more butter and sprinkle with more sugar. Repeat with the remaining 4 sheets of phyllo. Sprinkle the remaining 2 tablespoons gingersnap crumbs on top.
Spoon the apple filling onto the phyllo in a strip along the bottom edge, leaving a 1-inch border. Use the towel or parchment to help roll the phyllo into a log, starting at the bottom edge. Place seam-side down on the prepared baking sheet. Brush with more butter and sprinkle with more granulated sugar. Make a few slits on top with a knife to vent. Bake until golden brown, 30 to 35 minutes.
Beat the heavy cream in a bowl with a mixer on medium speed until soft peaks form, then gently fold in some of the reserved caramel (you may need to whisk the caramel first to loosen). Dust the strudel with confectioners' sugar and cut into pieces. Serve with the caramel whipped cream and more caramel; sprinkle with salt.