This was the best dinner we had in Vienna, and one of the best anywhere. The service was of the highest order. Unlike many European restaurants were English is spoken, at Vestibul the entire staff made it seem quite natural that they should speak English in order to enhance our dining experience (and one sensed the same might be true for several other languages). The waiter recommended some excellent Austrian wines to complement our food, including a sparkling wine (a Brubdlmayer) for dessert (much less expensive than the other dessert wines on the list).
The goose liver appetizer was silky smooth and outrageously delicious, and the sliced cheeks of angus beef were other-worldly. We steered clear of the signature lobster as recommended, but were knocked out by the veal meat loaf and the ragout of calves' lungs and hearts (traditional Viennese dishes). The desserts were also superb, in the usual restrained Viennese style (not overly sweet--or overly anything). And the room is absolutely gorgeous! For 125 Euro for two, this cannot be beat, anywhere!
Oh, it can be a bit hard to find. It is tucked away in a corner to the right of the entrance to the theater. You just have to know where to look.