I noticed in the new 2010 catalgue that the European Vista goes to Dachau...i dont have one handy right now so cant comment on the Discovery but you might be lucky
I'm not 100% sure about the Discovery, but we did visit Dachau on the European Inspiration this past July. I think the tour managers and drivers make a last-minute call depending on time. Ours told us about an hour before we arrived that he'd like to take us there since we have the time.