Melt butter in a large pot over medium heat. Add onion; cook and stir until softened, about 5 minutes. Add fennel bulb and zucchini; cook, stirring occasionally, for 3 minutes. Pour in chicken stock. Reduce heat to low and simmer soup, covered, until fennel is soft, 10 to 15 minutes.

Puree soup with an immersion blender until smooth. Season with curry powder, salt, and black pepper. Add white wine vinegar.

Chop reserved fennel tops finely and sprinkle over soup before serving.