Step 1
Make the vinaigrette: To a medium bowl, add the vinegar, garlic, and shallot. Season lightly with salt, then toss to coat. Slowly whisk in the oil until well incorporated, then adjust the seasoning to taste with more vinegar or salt if needed.
Step 2
On a shallow bowl or platter, arrange the heirloom and cherry tomatoes, and if desired, the cucumbers and onions. Season to taste with salt and black pepper, sprinkle with basil, drizzle with vinaigrette, and serve immediately.