@Richard2732 That makes sense. I currently have instant booking turned on for my listing in both VRBO and AirBnB, and so far this has not been an issue. However, I do have AirBnB set to add a day before and after each booking as a cleaning day (padding) and I think that this has prevented double or overlap bookings.
I had not noticed that AirBnB syncs only every two hours. It appears to me that when I get an AirBnB booking, AirBnB performs an immediate sync as I tend to see it right away on the VRBO side when I go there to verify that VRBO is also blocked out. Likewise with my VRBO to AirBnB calendars.
I don't think that leaving VRBO on a request only (not instant book) is an issue -- my VRBO guests tend to want to ask questions before they book, and they are also an older demographic, so a delay in my responding (when I wake up) has not been problem. I do switch over to request only on VRBO when approaching high season, just to be safe.