That seems odd. If you only the ARIA Player standalone application, can you load the Pizzicato Viola patch and play it on the on-screen keyboard? That would rule in or out a problem with the GIFF installation itself.
The Viola KS GIFF instrument uses the same keyswitches as the GPO Strings, so it should be F for Pizz (at the lowest octave).
Also, make sure that the expression map is using Note Velocity for dynamics; and check what the velocity values are. (Make sure the sound isn’t just really quiet!)