Thank you for your post.
Try creating a new user account on your Vista machine. Log in to that new user account, launch FileMaker Pro, create a new database file, and create a new account assigning extended privileges. Se if you are able to save those changes. If so, then you may have a problem with your original user account. You will then need to do some sleuthing to determine what is different between the two accounts. If the problem still occurs, then try uninstalling FileMaker Pro 11 and reinstalling. Try again saving the extended privileges.
Do you have anything else on your machine that might be considered out of the ordinary?