I know of a place here in North Carolina that builds converting equipment. www.pinnacleconverting.com Check out there site and give them a call. They can build what you need. You can get you roller blade knifes or use a razor type. Make sure you tell them if you want to unwind from wider core to slit adn rewind to a 6" core. If you are good at building stuff you can make one.
If you have a lathe, I've used a utility knife to cut down rolls of vinyl mounted on the lathe for my sign cutter.
Thanks guys I was thinking a portable unit to cut vynil out in the field as most of our jobs are off site.
I made a simple one in the past that held a roll of vinyl on a papertowel style holder and then attached a razor blade to it. I set the razor blade height then spin the roll by hand. Hard to explain, I don't think I have any pics of it though.