Puree the cucumber in a blender or food processor. Push through a fine-mesh sieve into a bowl.
Puree one cup of grapes and the garlic in the blender or food processor. Push through a fine-mesh sieve into the bowl with the cucumber.
Whisk the yoghurt, lemon juice, salt, vinegar and Tabasco into the soup.
Taste and adjust the seasoning. Cover and refrigerate for at least two hours or overnight before serving.
To serve, slice the eight grapes in half crosswise and scatter over the soup.
