I have a silhoulette (
http://www.silhouettemachine.com/). I think I paid $160 for it on a close out deal. (Mine is a first gen that does not automatically start the paper feed ... you have to crank it in before you start the cut.) It is limited in the width of cut to ~8". I have not used it to cut vinyl, but it definitely can be used for that.
The silhouette supports something called "print & cut". Basically, you print an image on a printer (can be an inkjet compatible vinyl), then load it into the cutter and it cuts around the printed image. They can do this because the cutter has a method of reading registration marks on the printout so that it knows where to cut.
To date, I have used it for cutting paper for my wife for her bulletin board at school and for pen plotting some CAD drawings that were ~30" long. They do not directly support cutting/plotting from a Mac, but there are ways to do it from Adobe illustrator and inkscape (free). There are plenty of options out there from the cheap to the expensive. If this is just a one-off, you might do better taking your design to a sign shop that has a vinyl cutter. I am sure that would be cheaper than doing the whole thing yourself.