Dear Mell,
Predictably, I would advise you to consult with a book and paper or object conservator experienced with skin treatment since vellum as a semi-tanned material is particularly susceptible to damage from moisture. If a humidification treatment is done badly it can result in a gelatinous mess! Determining the sensitivity of the ink wash and pigment is also important in deciding on an appropriate course of treatment.
Generally though vellum in good condition can be relaxed using indirect humidification. One of the common methods of doing this is to create a sandwich using damp blotter in conjunction with a non-porous membrane fabric that allows moisture but not wetness through. Examples of appropriate fabric include Gore-Tex and Sympatex. Both are sold at the preservation supplier Talas (although they are currently out of stock of Gore-Tex). There is a good amount of literature available online to explain the process as it is widely used for skin, paper and textiles:
– S. Dobrusskin, H. Singer, G. Banik – Humidification with moisture permeable materials http://iada-home.org/ta91_143.pdf
– S. Watkins – Practical Considerations for Humidifying and Flattening Paper http://cool.conservation-us.org/coolaic/sg/bpg/annual/v21/bp21-15.pdf
– Museum Textile Services – The Gore-Tex Humidification System http://www.museumtextiles.com/uploads/7/8/9/0/7890082/gore-tex_humidification.pdf
While these articles and other explain the setup, experience in knowing how long to humidify for and how to properly weight and dry the item is important. A yard of the membrane fabrics, while reusable is not cheap, and in conjunction with the other materials needed it might be not just advisable but also cost effective to have the piece treated by a conservator!
Best,
Rachael