Bring the quinoa and water to the boil in a saucepan. Reduce heat to medium-low, cover and simmer until the quinoa is tender and the water has been absorbed, about 15 to 20 minutes. Scrape the cooked quinoa into a large bowl and refrigerate until cold, about 1 hour.
Place the vinegar, salt, pepper, lemon juice and mustard into a blender. Drizzle in the oil while blending at high speed until the dressing is thick.
Add the cucumbers, pepper, red onion, tomato and olives to the bowl with the quinoa. Drizzle with the dressing and gently fold until evenly mixed.