When Ed Helms was cast as a grown-up Rusty Griswold in the new Vacation movie it marked the project out as a sequel by picking up one of the same threads later on, but also seemed to provide an option for a pretty clear new start. The typical "next generation" play.
Such follow-ups are never without room for callbacks and in-jokes of course. And that's really where the casting of Chevy Chase and Beverly D'Angelo fits in.
Variety say that the elder Griswolds are currently in negotiations to reprise their roles. The idea is that Rusty and his family will just make a quick stop by the folks'.
I'll be interested to see if this ends up being a "seqboot" with most of the plot points echoing the first film. The casting might say no, but I'd still bet on it. See also: The Evil Dead. Hell, see also: Evil Dead 2, for a while anyway.
I'd also bet on a reprise of Holiday Road. And that's a good thing.