Hello, I am a bit late getting back to this as I am somewhat new to posting.
I am not an expert, but I bought all of my tickets for opera and etc. directly from the source (i.e. Staatsoper, Oper an der Wien, Musikverein Jeunesse--rehearsal for an impossible-to-get concert.) At that time, all of the websites were in German, which made it a little tricky. Fortunately I am reasonably capable in German, because the online English translations didn't always make sense. The Staatsoper advance sales made me a little nervous. You sort of describe what you want, and buy them, and then later they send you a notice of what you've got, and you can take it or leave it. It did work out, and I got exactly what I wanted. For Zauberflöte on Christmas Day, I think we must have gotten the last three tickets available--separate seats up in the nosebleeds. I purchased tickets in October.
We ate our special dinner out on Christmas Eve. We got our reservation in mid-November for Gasthaus zur Oper/Plachutta. Lovely, delicious, very traditional. They even used Zalto wine glasses, which really elevated the wine experience.
On Christmas Day, we took it easy and ate food we had bought at a Heuriger a couple of days earlier, then went to the opera.
The whole visit to Vienna was magical.