Eddie has more than one daughter in the original Vacation. Only one couldn't talk. The other one could. Remember, she was talking to Audry about french kissing? It can be assumed that this is the daughter that's talking in Vegas Vacation.
Eddie has a different number of children in each movie. In Christmas Vacation, he only had two. And in the original he had like 5 or something. So, I don't think this needs to be made an issue.
Actually Eddie and Katherine started with 5 Children, the two from Christmas vacation would not have been born in the first movie. Katherine was pregnant though.Vicky in Vegas vacation was the older one from the original move,
Didn't Eddie say when they first got to Clark's house (this is in Christmas Vacation) that the older kids had stayed with some friends or something? meaning they probably had the same kids, just brought the younger ones with them. But another thing--how come Rusty and Audrey keep going from young to old and back to young again in each National Lampoons movie?! :O)