I've problems with Sibelius and dynamics, too. I use Sibelius 5.2.5, Vienna Ensemble, Vienna Special Edition and the latest SoundSet for Vienna Special Edition.
As you've said, Sibelius dynamics are over exagerated, so I've tried to use Velocity X-Fade with Expression. And I've also tried to change on the dictionary the values of every dynamic, but is very difficult to find the perfect configuration, because some values are good for strings, but very bad for the piano, and not the best for winds, and other values can be very good for piano, but not correct to use with strings instruments.
I've also noticed that if I change the Volume on the Sibelius Mixer, it changes strangely the velocity sent to Vienna Ensemble, not just the volume. You can try it: write any music with a forte dynamic, and then playback changing the volume at the Sibelius Mixer. You'll can hear that not only the volume changes, also changes the type of sound, the attack of the note, the quality.
So, what volume we must assign to each instrument on Sibelius Mixer? 100 to all? Or better the maximum?