I would recommend you buy one at your local drugstore and try it out in the parking lot rather than order one online. I won one of these things in a Yankee gift exchange this year and discovered that it's straps aren't long enough to stretch all the way around the VX's seat (the side supports are just too wide).
I'm sure there's something out there that fits, but I don't think the manufacturers list "strap length" on the box so it's likely to be trial and error.