Between Vinca and Dieffenbachia, which genus has range that extends further north?
Vinca: Vinca ( ; Latin: "vincire" "to bind, fetter") is a genus of flowering plants in the family Apocynaceae, native to Europe, northwest Africa and southwest Asia. Dieffenbachia:  It is native to the New World Tropics from Mexico and the West Indies south to Argentina.
Vinca