We use homeaway and vrbo often. We have always been happy, but I usually have a back up plan incase, so I understand where you are coming from. You can get vacation rental insurance thru vrbo and homeaway. Homeaway use to offer for free, now it is about $39. In ST there is Esencia, which has 1-2 bedroom cabinas with a basic kitchen and the benifits of a hotel. In a pinch, if the house you book in ST is really bad, you could always run over to Esencia and see if they have any places available.
Have you booked vacation rentals before? Some people expect the same type of service you get from a hotel. Also things happen in a house, appliances breaking, leaking plumbing. Just like at home. It isn't like they have a bunch of hotel rooms to move you into, so if something breaks you will probably have to deal with it and hope the owners are fair. I just got back from a trip. We rented a condo and loved it. Another poster was also renting a condo from the same owners and had a fit that one of the two TVs didn't work and the wifi was iffy. Everything else was top notch and high end. The condo rentals were 1/2 the price of what a hotel or suite would have cost in the same location, but some expect the same type services no matter what they book. I guess what I am trying to say, is if you are looking for perfect, a vacation rental may not be for you. It is difficult to complain and get a quick resolution if there is a problem.