Visible crew/equipment: On the second roller coaster in Wally World, as they begin leaving the station and going up the lift, crew can be seen crouching in the background trying to hide.
Visible crew/equipment: When they leave dead Aunt Edna on the patio at the family members' house. While the family is talking you can see the mic above their heads going back and forth when each cast member is speaking.
Visible crew/equipment: 2 crew members are visible operating the pirate ship ride.
Visible crew/equipment: When Aunt Edna tells Eddie "see I told you he wouldn't like them", Clark closes the door in her face. A film light is reflected in the center of the frame.
Answer: According to the commentary from "Christmas Vacation" this is an inside joke referencing the fact that Clark is away from home so much with work, he hardly even recognizes them anymore. "European Vacation" could not obtain the same kids who played in the original Vacation, namely Anthony Michael Hall, and tried to cast kids whom looked close. Thus the joke began. No doubt the actors' aging between movies also plays a part.
Jazetopher