Place 1/4 cup vinegar, the mustard, and 1 teaspoon salt into a small lidded container and shake vigorously to combine. Add the olive oil and shake to combine. Pour the dressing over the chickpeas and toss to combine. Spread the chickpeas on a half-sheet pan and roast until the chickpeas are deep golden brown and crunchy, tossing every 15 minutes, for 60 to 65 minutes. Remove from the oven, return to the mixing bowl, add the remaining vinegar and salt, and toss to combine. Transfer back to the sheet pan and spread out to cool completely before serving as a snack.