For assistants, we can currently do this by setting the desired server events in the assistant's configuration, specifically in the Advanced tab when editing an assistant (select which server messages (events) you want to receive, such as "conversation-update", "end-of-call-report"). There is nothing in the documentation about how/if this can be done for Workflows.

Currently supports server event subscriptions like conversation-update, end-of-call-report only for Assistants, not Workflows for Workflows, you must manually send data using an API Request node before the call ends.. https://docs.vapi.ai/api-reference/assistants/update