I list our villa on VRBO, and my understanding is that an owner has an option to choose online booking, in which case VRBO does not charge the owner an annual LISTING fee, but will charge the owner a "commission" for each booking, and also charge the renter a processing fee. The other option for us owners is to continue to pay an annual listing fee and not sign up for online booking, in which case there is no fee on either end. That is the option I will choose when my renewal comes up, but by not choosing the online booking option, I will drop down the pecking order of my listing being seen by potential renters. C'est la vie.
For the record, VRBO is owned by HomeAway and TripAdvisor is owned by Expedia.