Have you looked at "Settings" then "Surcharges & Discounts"? You should be able to set a % discount just for that property, and just in VRBO, for nights "Greater Than or Equal To" X number of days. This talks about it: https://www.ownerrez.com/support/articles/length-of-stay-discounts
As another option, from within VRBO you cannot do "monthly rates" but once you set your nightly rate you can turn it into a monthly rate by giving a "discount" for those who book for a month or more. Go into VRBO, to the rates calendar, and click on the "rates settings" tab at the top, right. There you can put in whatever the % is to discount your nightly to what you want for a monthly. VRBO defines monthly as 28 nights or more.
