@Hannah1249 How soon after booking on AirBnB did you find the home on VRBO? If the calendars are linked and synchronized, it should have shown as booked after about 2-3 hours, but not immediately.
If it has been 24 hours or more and the home is marked booked on AirBnB and still marked as available on VRBO, I would contact the host via VRBO and ask if your booking via AirBnB and AvantStay has been communicated to them.
It appears that this listing is "new" on the AirBnB website. Perhaps the person at AvantStay has made an error with the listing price when they created the listing.