Combine vinegar, brown sugar, ketchup, chili flakes, and water in a small saucepan and bring to a simmer over medium-high heat, whisking to combine until sugar is dissolved. Combine remaining tablespoon water and corn starch in a small bowl and whisk with a fork to form a slurry. Whisk into sauce and bring to a boil. Allow to cool while you form the crab rangoon.
Combine cream cheese, crab, and scallions in a medium bowl and fold with a spatula or your hands until combined. Lay one wonton wrapper out on a cutting board (keep the rest under plastic wrap or a damp towel) and place a small amount of filling in the center (about 1 1/2 teaspoons). Moisten the edges with a wet fingertip, the seal by either folding it in have in a triangular shape, or by pushing the four edges in towards the center to create a four-pointed star. Be careful not to allow any air to remain inside as you seal them. Transfer to a large plate, cover loosely with plastic wrap, and repeat until all of the filling is used.
To finish the sauce, add pineapple chunks and puree with an immersion blender or by transferring to a standing blender.
Heat oil in a wok or Dutch oven to 375°F as registered on an instant read thermometer. Carefully add 10 to 12 crab rangoons to the oil. Cook, adjust flame to maintain temperature and agitating and flipping them constantly with a wire mesh spider until crisp and golden brown, about 3 minutes. Transfer to a paper towel-lined bowl to drain, season with salt, and serve immediately with sweet and sour sauce.