My current followers are Vilja and Cerwiden, which I like and I would like to keep. Will they be compatible with the new area or do I have to dismiss them when I start the DLC?

Unless any new content breaks the scripts associated with them, you should be good.

If there are portions of Dragonborn that have a forced follower, you may run into trouble. If you don't already, I recommend using UFO (Ultimate Follower's Overhaul), which allows you to have multiple followers. The author is pretty good about updating as new patches/DLC come out.