I haven't closed the vault doors yet and was wondering if I could drop misc. items in that room without them disappearing/resetting.

http://www.uesp.net/wiki/Category:Skyrim-Places-Safe you can find a complete list of all the safe locations in the game.

If i recall the door locks itself eventually so that may be a bad idea. However you can force it open by getting a follower to stand in front the door and running into him/her

Edit: Also uesp says the cistern area resets in 10 days

^This. The doors will lock by themselves.

Figures. I appreciate the help.