Bring quinoa and water to a boil in a saucepan. Reduce the heat to medium-low, cover, and simmer until quinoa is tender and water has been absorbed, 15 to 20 minutes. Transfer cooked quinoa to a large bowl and refrigerate until cold, about 1 hour.
Blend vinegar, Dijon, salt, pepper, and lemon juice in a blender. Slowly pour in oil, with the blender running, until dressing is smooth and thick.
Remove quinoa from the refrigerator. Add cucumbers, tomatoes, olives, bell pepper, and onion. Pour dressing over top and gently mix until evenly combined.