Mix corn, black beans, pinto beans, black-eyed peas, red bell pepper, yellow bell pepper, orange bell pepper, and red onion together in a large bowl; stir in vegetable oil.
Put granular sweetener into a bowl; stir vinegar into sweetener. Mix water into sweetener-vinegar mixture until sweetener is dissolved. Pour mixture over corn-bean mixture; toss to coat. Cover bowl with plastic wrap and refrigerate 8 hours to overnight. Drain liquid from mixture in a colander before serving.