First of all, if you haven't done so yet, I strongly recommend updating to Sibelius 5.2.5. Some playback bugs have been solved in this version.
The sound set "VSL Ensemble Special Edition" should be fine. "VSL Special Edition" is an old version. You can delete it from your Sounds folder.
For dynamic changes Sibelius sends CC11 (MIDI Control Change 11) values. These values by default are triggering the Expression slider in the Vienna Instruments. The volume should change a bit, but the timbre will not. In order to get more realistic crescendi and diminuendi you can do the following: Go to the Perform page in the Vienna Instruments. Activate Velocity X-Fade. Then Click on Map Control. Now as source for Expression set none. Then go to Velocity crossfader and as source select ControlChange/Controller 11. Don't forget to save your Playback Configuration in Sibelius after that.
The velocity of single notes can be adjusted in the Playback Section of the Sibelius/Properties window. In your case I guess you ment the general volume of the Trumpet staccato notes. You can lower this volume in the Control Edit section of the Vienna Instruments. Click on the staccato patch and then on the "1" underneath the matrix. Now the very right slider you see is for the volume of the patch you selected.
Andi