One other idea came to mind. A discount could be offered to a tenant for each reference that could be provided by past home owners. For example, Lets say that a renter has rented from a handful of home owners on the VRBO site in the past. For each reference that can be provided, a further discount could be offered. For example:
Each reference from a current VRBO home owner nets the renter a $10 discount.
The renter simply provides the home owner with the VRBO property number which hosted their previous stay, and if it can be verified by the home owner...the discount is given. You could limit the offer to a maximum number of references, to keep the costs perfectly in line with your expectations.
I know speaking from experience that I would gladly hand over $20 to the renter if he/she could supply me with 2 VRBO references which I could verify myself.