Combine cucumber and vinegar; set aside. Combine breadcrumbs and next 4 ingredients. Combine water and egg whites, stirring well with a wire whisk. Dredge tomato slices in breadcrumb mixture, and dip in egg white mixture. Dredge in breadcrumb mixture again.
Heavily coat a large nonstick skillet with cooking spray. Place over medium-high heat until hot. Add tomato; cook 1 minute on each side or until golden. Serve warm with cucumber mixture.