The possibility of personalizing devices and online services is important for end users living in smart environments, but existing End-User Development interfaces in this field often fail to provide users with the proper support, e.g., because they force users to deal with too many technological details. This paper explores novel approaches for personalizing IoT ecosystems via natural language and vocal interaction. We first conducted seven interviews to understand whether and how end users would converse with a conversational assistant to personalize their IoT ecosystems. Then, we designed and implemented two prototypes to define trigger-action rules through vocal and multimodal approaches. A usability study with 10 participants confirms the feasibility and effectiveness of personalizing the IoT via voice and opens the way to integrate personalization capabilities in smart speakers like Google Home and Amazon Echo.