Nice review - thanks for sharing!
I was also really curious about the the older Vienna Imperial and ended up purchasing it during VSL's holiday sale this past November, but found that I much prefer the newer Synchron Bosendorfer Imperial in almost all respects, so ended up returning it after about a week.
I really liked the fact that it had real una corda samples, but it didn't appeal to me as much from a sound perspective. I didn't gel with any of the preset mic perspectives, and didn't feel that the recording depth - while good - was quite on par with the newer Synchron pianos. Tuning also sounded a bit off in certain areas. Playability was decent, but didn't feel as realistic to me as the Synchron pianos, especially in regards to the pedaling.
The Vienna Imperial player also had some weird stability issues w/ my Mac and kept crashing unless I set the voice count quite low, which is strange given that I don't have any similar problems with the much more resource heavy Synchron engine. I haven't heard anyone else report similar issues, though, so I'm assuming it's something isolated to my particular setup.
But even if I had managed to work out the technical issues, I just didn't see myself playing it that often, so I ended up returning it and getting the CFX instead, which I've really come to love - it's probably become my favorite of the Synchron pianos along with the Steinway.
I've also been demoing the new VSL Fazioli for the past several weeks, which is proving to be my least favorite of the Synchron series, but I'll reserve my comments on that one for a separate thread.