California Veggie Sandwich

Name: California Veggie Sandwich
Ingredients: 2 cups apple cider vinegar; ½ cup (packed) light brown sugar; ¼ cup kosher salt; 4 cups any combination shredded carrots, sliced cucumbers, sliced red onions, and/or sliced mild fresh chiles; ¼ cup buttermilk; ¼ cup plain whole-milk Greek yogurt; 1 tablespoon fresh lemon juice; 6 tablespoons olive oil, divided; Kosher salt, freshly ground pepper; 2 ripe avocados, halved; 6 ounces fresh goat cheese; 6 cups mixed lettuce leaves, ribs removed if thick; 8 slices multigrain bread, toasted; ½ English hothouse cucumber, thinly sliced on a diagonal; 2 cups sprouts; 2 heatproof 1-qt. glass jars