I have a hard time putting aside all forms of common sense when watching movies. Picking apart all the reality flaws of the movie "Waterworld" has become something of a past time for my brother and I. National Treasure had several of the same types of flaws.
For one, Ben Franklin was supposedly part of this whole Mason/Knights Templar conspiracy to hide away such a vast fortune and keep it secret. OK. On the subject of keeping secrets Ben Franklin once said something to the effect of "The maximum number of people who can keep a secret is three, provided two of them are dead." How many people would it have taken to construct the underground vault where the cash was kept?
Do torches readily light after sitting on a wall for 200 years?
Where were all the insects and rodents who certainly would have made such an underground cavern their home?
Why was the air still breathable in a stagnant, sealed, deep sub-surface chamber? Or, if ventilation was provided, why was the area not damp, moldy, and slimy?
How many times can a document over 200 years old be rolled up, stuffed in a tube, pulled out and examined, and rolled back up again before it crumbles? One of the big concerns with the Declaration is that time is taking it's toll on the parchment, so why was the movie version to damn durable?
If your going to get so far off the reality plane, throw in some space aliens, phase plasma rifles, and black magics just for kicks and giggles, otherwise, keep the plot on the ground.