Dana Barron is the first (and to-date only) actress to reprise her role as Audrey Griswold. The Griswold kids are played by different actors/actresses in each of the "Vacation" movies.
According to Randy Quaid, many of Cousin Eddie's characteristics (most notably the clicking of the tongue) were based on those of a guy Quaid knew from his Texas childhood who had similar traits.
Beverly D'Angelo was asked to reprise her role of Ellen Griswold in this sequel, but had to turn it down to take care of her children in New York City.
"Third" played by Jake Thomas, is presumably the child conceived by Eddie and Catherine during the events of Vegas Vacation (1997).