Booking.com - Booking.com requires Instant Booking for all listings in all scenarios, whether they are managed by software like MyVR or not.
Airbnb - Airbnb is different - they do not require instant booking if the listing is managed directly through the Airbnb dashboard; if the listing is managed by software like MyVR, then they do require instant booking.
FlipKey/TripAdvisor - Instant booking is not required by Flipkey, but it can be enabled if you have a Pay-Per-Booking type subscription and request it from your FlipKey/TripAdvisor account manager.
HomeAway/VRBO - Instant booking is not required but it can be enabled if you turn it on within your booking policy settings in MyVR. Learn more here.