Puree the tomatoes, cucumber, bell peppers, grapes, parsley, red onion, garlic, vinegar and cumin in a blender until almost smooth. With the blender running, pour in the olive oil in a slow, steady stream until the gazpacho is smooth.
Add 1/2 cup vegetable stock and blend again. If the gazpacho is too thick for you, add more stock until you achieve a consistency that you like. Season with salt and pepper and chill in a pitcher or bowl, about 1 hour. Drizzle each serving with olive oil and top with parsley.