Place shallot in a small bowl, and drizzle with sherry vinegar; let stand 30 minutes.
Toss together tomatoes, cucumber, bell pepper, and salt in a medium bowl.
Pour tomato mixture, shallot mixture, and vegetable juice into a blender
Pulse until finely chopped, about 4 pulses.
Process until smooth, about 1 minute.
With blender running, slowly drizzle in oil.
Season with additional salt to taste.
Chill, covered, until very cold, at least 1-2 hours.