I love Vienna Imperial. I agree with Lylo that this is the VSL Bösendorfer to go for and not Bösendorfer Imperial, which isn't really aimed at pianists. There are some problems with Vienna Imperial, and I would probably wait to see what the Synchron version is like.

The playability and responsiveness of the VSL pianos is fantastic.

Since there is no Bosendorfer Imperial model in pianoteq, I bought the famous Bosendorfer Imperial VST in VSL.
But it turned out to be stupid - worse than a bosendorfer fxp in pianoteq based on another piano.
So pianoteq really has to make the Bosendorfer Imperial!!!!

I still hope (and expect) that Modartt launches into such a preset (probably under a slightly different name). The team is now mature enough to make a Viennese piano.
When a few years ago I looked for a good vst for piano, I first listened attentively to the demonstrations of VSL, but it is Pianoteq that I chose, and which is still much improved since, choice that I do not regret. VSL vst does not seem to be a bad midi player, but Pianoteq is a real instrument to be played on the keyboard (not just a jukebox).