Place Brussels sprouts into a saucepan, pour in about 1/2 cup of water, and bring to a boil over medium heat. Cover, and cook sprouts until hot, about 10 minutes. Remove from heat, and drain.
Mix together the apple cider vinegar, vegetable oil, salt, and black pepper in a bowl; add the cooked sprouts and stir to thoroughly coat with dressing. Refrigerate until chilled, at least 2 hours. Just before serving, mix tomato into Brussels sprouts mixture; sprinkle with parsley.