I think there's an easier way.

Go here: http://mayang.com/textures/Nature/html/Mud/index.html

And download one of the cracked mud textures, they look like scales, or see if you can find a reptile texture on the web. The good thing about mayang's textures is that they are so big that you don't need to make them seamlessly tile (normally).

But if you do need a seamless tile fill, then make one using a paint program.

Make your vellum parchment as normal.

Overlay it with a new layer using the fill and coloured to suit - try using 'multiply' and 'overlay' and playing with the opacity settings.

Put your symbols / drawings etc on layers above these.

Hope this helps!

Ravs