Question: How did Gomez and Fester Addams get back up to the first floor from the vault? All they enter was the slides.
Question: It is revealed at the end of the film that Gordon is in fact the true Fester. However, all the photos and Gomez's home movie show young Fester as being completely bald. Then how did Gordon manage to have a full head of hair which he had to shave off in order to initially pose as Fester?
Answer: I guess we assume he always shaved his head.
Answer: Presumably when Fester was lost in the Bermuda Triangle, whatever supernatural forces caused him to lose his memory also made him grow hair. This gets reversed at the end of the movie when he's zapped by lightning, restoring his memories and destroying his hair follicles as well.
Question: When Gordon and his mother are in Tulley's office, how does Tulley see a resemblance between Gordon and Fester? I thought he had not been seen for 25 years.
Answer: He's seen pictures and paintings of young Fester and notices a resemblance. Like you said, no-one had seen him in 25 years, so all that was required was a passing similarity.
Answer: No doubt the Addams house had one of many secret, confusing passageways to get to the higher floors (and other parts of the house) from the vault that the viewer obviously is not privy to.
Scott215