Other than at Jedi training... I've never seen Vader walking around Disneyland.
Often, you will see a pair of Stormtroopers wandering around Tomorrowland (usually in front of Star Tours), but I've never seen Vader.
I'm sure he has to report back to the Emperor, on a regular basis. Also, over-seeing the construction of a Death Star probably keeps him quite busy. I doubt he has time to 'mingle' with rebels like us...
If the Jedi Training Academy gets cancelled for whatever reason (weather or malfunction) they do substitute a meet and greet with the Jedi, Maul, Vader and the Stormtroopers where you can get photos, but no autographs.
This was told to us by one of the Academy Cast Members after asking about meet and greet opportunities last February.
Peter Pan: "You know, your hair is on fire."
Me: "Yes. It happened after a recent trip to Neverland."
Peter Pan: "Well, you can dunk your head in the pond behind us, if you want."
Lost Child of Neverland since 1996. ~Favorite Hero: Peter Pan. Favorite Villian:Malificent.Favorite Ride: The Haunted Mansion.~
I thought he stayed out for meet and greet after the Training Academy? Maybe it's just the storm troopers. I know for sure I've seen troopers taking photos with guests after a couple shows, but it may only be for a short time.
I have never seen Vader walking around, even when the Jedi Academy was not running, but I have seen Stormtroopers quite a few times. I noticed they usually stay on the side with Space Mountain and Star Tours and don't venture too far.