Whisk 1/4 cup cider vinegar, 2 tablespoons each vegetable oil and mayonnaise, 1 tablespoon sugar, 2 teaspoons dijon mustard and 1 teaspoon celery seeds in a large bowl. Add 1/2 head shredded green cabbage and 1/2 bunch shredded collard greens; toss well. Season with salt and pepper. Cut 1 Granny Smith apple into matchsticks and toss with the slaw right before serving.