<< Whenever the President addresses a joint session of Congress a member of the administration is kept at an undisclosed location in case of a tradegy that kills all in the buidling. Typically it is not the VP but since we are at war it is wise to put #2 someplace else. >>
It's the same thing as when there is a State of the Union...the Speaker (3rd in command) is separated in case of a disaster...to keep the government running.