I too had a bad experience with a vacation rental and was unable to post my review to VRBO. That's why I founded Dwellable, a vacation rental site. Here's the full story on GeekWire from a few months ago:

http://www.geekwire.com/2011/hawaiian-beach-vacation-hell-sp...

This behavior on the part of HomeAway (VRBO) is really a natural consequence of their business model. When the property manager is your only customer, it doesn't make sense to post negative reviews.

We're doing things differently at Dwellable, of course.