Cut the top third off each lemon. Cut a small slice off the bottom of each lemon so it stands up. Using a spoon, carefully scoop out the inside of each lemon, creating a lemon cup. Put the lemon cup in the freezer for at least 1 hour.
Put the sorbet into the bowl of a food processor. Add the limoncello and vodka and process until smooth. Place the frozen lemon cups onto shot glasses filled with limoncello. Fill the lemon shells with the sorbet mixture and serve.