Vocus.io supports the concept of a "hot lead", which is a label that is automatically assigned to your contacts based on their behavior. This feature is only enabled when there's a webhook configured under your profile; it is meant to proactively inform you about noteworthy user behavior.
Vocus.io captures two behavior signals from your leads: email opens and link clicks. From your Account screen (Dashboard > Settings > Inbox), scroll to the end and look for Webhooks. The last webhook in the page is a "Hot Lead" webhook, which allows you to trigger a special workflow whenever a user is assigned the Hot Lead label.
You can customize what defines a hot lead from the drop down menus underneath the webhook field: number of signals and type of signals. For example, you might want to trigger a phone push notification after a lead opens your email or clicks your links more than four times. See this page for a detailed tutorial.