I had vellum invitations, I'd have to dig to find them, its been a few years... one thing I would suggest is that if you are planning on printing them out yourself you might want to just print out on plain white paper and have it copied onto the vellum because all of the computer printers we tried (bubblejet, inkjet, laser) smeared on the vellum because it really doesn't absorb the ink. 3 cent copiers seem to do a good job with not smudging (saves ya from buying a new printer cartridge too!)
You could probably put two invites on each piece of paper, cut it down smaller than the cardstock and use a bow or sealing wax to attach the vellum to the cardstock. I'd also look at scrapbook magazines for vellum ideas. Good luck with the planning!