@Haru139
It's tough for anyone to get a handle on what is happening without being able to see your actual calendar on the Host side. The Host controls the price on any given date on the calendar, so not sure what you mean by the guest was able to book at a very low rate in December? Are you using Smart Pricing?
If you cancel a reservation, there are penalties applied by Airbnb and they can also block your calendar for the dates you cancelled.
Cancellation Policies
https://www.airbnb.com/help/article/990
...In addition to a cancellation fee, other consequences may apply, such as preventing the Host from accepting another reservation for the Listing on the affected dates by blocking the Listing’s calendar.
Hosts who cancel confirmed bookings without a valid reason may experience other consequences, as explained in our Terms of Service and ground rules for Hosts. For example, Hosts may have their Listing or account suspended or removed, and may lose their Superhost status..."
Vrbo Sync
If you still have access to the calendar, are you not able to just delete the sync to Vrbo on your Airbnb calendar? You can also try deleteing the sync to Airbnb from your Vrbo calendar. That would enable you to have your Vrbo calendar open. If you are using Instant Book, suggest you turn off Instant Book on Airbnb while you are trying to resolve this to prevent a double booking (if you haven't done that already).