Vellum is easily aged using tea as an agent. That is what I used for my DOI replicas as well when I did the Narnia maps. Thinner vellum has a almost transparent quality, thicker vellum is darn hard to find but it is what you want. The thinner vellum also on a large document tears fairly easily when wet, something else to consider when doing an accurate replica.
Remember as well, if you are doing a replica like that from The National Treasure movie, that replica was not accurate to the real document. They enhanced the lettering so it could be seen on the screen. The current document is badly faded.
The replicas I made for the board long ago were on vellum and used the archive scans to produced the enhanced look as well the exact duplicate of the current document.
Size of the signed original Declaration of Independence in the National Archives in Washington, DC, is 24 1/2 inches wide and 29 3/4 inches high. Not many replicas get this correct.
A copy was made of the original using a process of lifting the ink off the original to produce a printing plate at the US mint. This was done in the early 1900's and it took away most of the ink giving the original the vastly more faded look. They did not consider at the time the issue of preservation as much as making a new limited print for the public at the time. That plate still exist and was used to make another extremely limited edition back in the 90's which sold for $ 250.
There has been talks about what to do when the original document fades away completely including re-inking or just printing a new document to display by the totally faded original. There has been no final decision on what to do at this time. It is expected that it will be totally unreadable in the next 100 to 150 years.
Downloadable pdf of the original documents can be found here:
http://www.archives.gov/exhibits/charters/charters_downloads.html
