In many situations, property managers get a booking that they later need to attribute back to a channel. A simple example for this case if a migration from another PMS where you need to flag bookings with channel attribution after the data import.
For Vrbo, the way to attribute the channel source is done by editing the booking, and from the Data tab, just click the box "Attribute this booking to Vrbo", as below:
Attributing is to be done if the reservation should exist in Vrbo but doesn't. If the reservation already exists in Vrbo, attributing the manual lead in Hostfully to Vrbo will result in Vrbo double-charging the commission on your end.