Hi Grigoris,
i am a bit confused but i will try to help you:
if you are looking for an easier way to manage volume levels between articulations you might be interested into the 'Advanced' view of Vienna Instruments (or Pro). As you can see in the screenshot there's a fader right above the A5 Key in the virtual keyboard, that fader controls the volume for that actual cell in the matrix, so you will always be able to set different volumes for each articulation, whether you are planning to use multiple MIDI tracks for each instrument or just one MIDI track for each instrument (therefore keyswitching between articulations).
I am afraid we can only set this levels by ear, i might be wrong though.
I hope this helps.
Best Regards
Francesco
Francesco