You know, something different, yet attractive. Look for a high-rag content laid paper. Even if you are, you could enter it as a 14th century copy of a poem written before paper was introduced to Europe. If that's the case, I'd just use a good, attractive paper. No point in hammering a square peg into a round hole.
Paper has it's own unique beauty and charm.
No need to make it try to be something it's not. There's also a purely technical consideration. Real vellum would cost you between 100 and 200 times that much, depending on where you got it. If that doesn't daunt you, get in touch with me next time, but give yourself a couple months time to work out the logistics of delivery. Arte Editions, 1999.