Vibrato for non vibrato instruments. Knob doesn`t seem to work. What is wrong?

First, be aware that there may well be some instruments in Garritan Libraries that do NOT support a vibrato LFO at all.  If there is a specific instrument from a specific library to which you are referring it would be helpful to know this. 

If the instrument has a "Vibrato Speed" knob in the control panel (I think it's usually CC17 in Garritan Libraries), then you'll need to use 'after-touch' to engage 'vibrato intensity'.

If your keyboard supports after-touch, pressing down on the keys once they bottom out should case vibrato to fade in.  The harder you press, more intensity is added to vibrato, while the Vibrato Speed dial lets you control how quickly the LFO oscillates. 

If your keyboard doesn't have an after-touch pressure strip, then check its manual to see if it has an alternate means of sending after-touch (Assigning it to a fader/pot/wheel/joystick/cont. voltage pedal/etc.).  I.E.  Some boards with programmable faders/pots/etc. may have after-touch listed as events that can be sent.  Some might simply have unusual CC listings that seem outside of the GM Protocol you could try [Anything above 127 or 128... I.E:  CC131 means after-touch for some brands of MIDI controllers])

If you know for certain an instrument in ARIA supports a vibrato LFO, and your keyboard does not have a way to send these 'after-touch' commands then you might be able to force software to fix things.  Typically you'd just draw after-touch events onto a controller lane in your DAW.  If you need it during 'live playing' you might could use transformers in your DAW to 'convert' some other CC into 'after-touch' in real time. 


There are also stand alone utilities that can help:  Bome MIDI Translator or Bidule are great examples of utilities that can capture and 'transform' MIDI events in real time.

Finally, as a last resort:  You could edit the sfz file for an instrument to change the CC (continuous controller)  that can control the vibrato LFO.  Here is a thread on this topic:  https://makemusic.zendesk.com/hc/en-us/community/posts/115000488188-Using-After-Touch-during-performance-on-my-midi-Keyboard