According to Randy Quaid, many of cousin Eddie's characteristics (most notably the clicking of the tongue) were based off a guy that Quaid knew from when he grew up in Texas years ago who had similar traits.
When shopping with Clark Eddie asks Clark if it was his company that "killed all those people in India". He is referencing the Bhopal disaster, also known as the Union Carbide disaster in which leaks from a Union Carbide pesticide plant escaped into the air. Thousands of people died and many more were sickened.
After Clark Griswold (Chevy Chase) unsuccessfully attempts to demonstrate his handiwork with the house Christmas lights to his family, he asks his son, Rusty (Johnny Galecki), to help him check all the light bulbs again. Rusty looks at his bare wrist, pretending to have a watch, and excuses himself. Looking at a bare wrist and pretending to have a watch is one of Chevy Chase's trademark gags.
In the scene when the police storm into the Griswolds' house, the song "Here Comes Santa Claus" sung by Gene Autry is used for the background music. Randy Quaid (cousin Eddie) is the third cousin of Gene Autry.
The movie is based on John Hughes' short story "Christmas '59", the second Vacation story to be published in National Lampoon's Magazine (the first was "Vacation '58", which was the basis for the first Vacation movie). The Christmas story was printed in December 1980. The label on the home movie reel that Clark finds in the attic is labeled "Xmas '59," a further allusion.
Cousin Eddie has a son named Rocky wearing a Las Vegas shirt. Coincidentally, footage from this film appears in the film Rocky V (1990). Furthermore, the next film in the series would be Vegas Vacation (1997).
Just before Clark gets locked up in the attic, he pulls out an old present from a hidden slot, and it contains a card that reads "Happy Mother's Day 1983, Love Clark". The first movie, Vacation (1983) was released that same year.
The Warner Bros. back lot used for the neighborhood in which the Griswolds live is the same as the one used on the TV series, The Middle (2009), which featured Brian Doyle-Murray (Mr. Shirley) in a recurring role as Mr. Ehlert for the first three seasons.
When Clark shows his coworker Bill the brochure about the swimming pool he wants to put in his backward, he is seen drinking coffee out of a Tasmanian Devil coffee mug. Miriam Flynn (Cousin Catherine) would later go on to play the voice of Taz's mother in the cartoon Taz-Mania (1991).
The trivia items below may give away important plot points.
The scene where the cat bites on the Christmas lights cord and gets electrocuted was nearly cut from the movie. Prior to the first test screening. the studio execs wanted the scene taken out, fearing that it might offend some viewers, but producer Matty Simmons begged them to leave the scene in, and they eventually gave in to his request. After the first test screening, the test audience had scored the cat electrocution scene as the No. 1 favorite scene throughout the entire movie.