Overlay with Exchange calendars and SharePoint farm calendars
First at all, you need to go to Virto Calendar Settings page.
Then click “Available calendars” and select your current calendar or create a new one. Click “Edit”
In the middle of the calendar settings page check the box to display events from Exchange Online Calendar or/and Exchange Online Calendars.
Save the changes.
Now you can see the new button on your calendar page.
Click it to display Exchange calendars and all SharePoint calendars from SharePoint farm.
If you use Internet Explorer as a default browser, you need to add Calendar URL to the list of trusted sited in the browser settings.
Press “Connect to Exchange” and confirm the calendars overlaying in pop-up window, if your browser did not confirm it automatically.
Reload SharePoint page to display Exchange Online Calendar on Virto Calendar.