Is it just about her not leaving Collinwood for 18 years?
Yes, that, and that it coincides with her father's departure, and Carolyn surmises that it's in case he should return.
I thought she had already been told about that.
At this point, Vicki has been told that she's the first stranger in the house in 18 years, but I believe she's hearing for the first time that Liz stayed put during that entire time. The audience, however, has already learned it from other characters.
BTW, MB has provided links in the history area that coincide with each of the day's highlighted episodes and will take you directly to the Robservation. I highly recommend using the links because finding those summaries at a later date will pose a bit of a challenge.