I was going to edit my post, but I don't think that would make a notification. I've been thinking about Luke 1:28 since you asked about it, and I realized that the vocative form doesn't add up because χαιρε isn't a noun. So I got to thinking, and it is actually a present active indicative verb, which still translates to "rejoice!" but I would be bugged by the fact that I had it wrong before.