I honestly suggest no motor at all, implement a pneumatic system (If you are not currently implementing one...) and add an in line vacuum generator. Here's an explanation of how they work...
http://store.workshopsupply.com/cata...or-p-2537.html
I suggest Piab, good brand!
http://www.piab.com/en-US/Products/V...l-49-108-scfm/
Now, we've never used them on a robot, so I don't have any experience to share...but I've done plenty of research as well as having a father working at a shop that uses them, to know where to turn next time my team asks for a vacuum system.
just my 2 cents!