Just to plug VRBO some more... we haven't used it for Vegas yet but we've used it to find accommodations for our other trips (like Oahu & Maui) and have been very successful.
To be honest, I hadn't heard of the Signature before this thread but I looked it up and it looks very nice. Also highly rated on Tripadvisor. Just doing a little more digging around... suites for the Signature can also be found on other "vacation rentals by owner" websites such as Craigslist and Homeaway (which we have also successfully used). Pretty similar prices compared to ones posted on VRBO.