We did the VIP tour one time at Disneyland and we did tip.
The reason being, is I'm pretty sure that while Disney charges a fortune for the tours, the VIP cast members probably don't see much of that money.
I apologize as I don't remember how much we tipped her. It was good when compared to hourly wages but probably not commiserate with the total cost of the tour. We made sure that every time we stopped for a snack or whatever we bought her one, a bottle of water etc. and at the end of the day we were grabbing a quick service meal before the evening parade and asked her if she would like to join us, and she took us up on that as well.
It was fine because she was an amazing guide who made our day truly memorable. She asked if we were interested in the history of the parks/ rides etc. and when we said yes, the entire day told us stories and tidbits. It was great!