I will say that this is only idea to try, not know that this is what you need to do, and it will be very exciting to keep replying for the results you have! Aren’t we altogether much smarter then one?
I have found in Cubase SX that you can control a VST instrument with MIDIBOX64.
Here is a description what you need to do and test:
Under the File/New Project choose ‘Empty’ and choose the directory where you’d like to store the project files.
Under the Project/Add Track choose ‘MIDI’ track.
Under the Devices/VST Instruments choose the Steinberg’s “a1”.
under the in port choose the MIDI in port, and under the out choose a1.
Now click on the small ‘e’ icon in the Inspector window under the ‘Channel’ tab and the brown&green colored window will open(MIDI channel). The first 4 windows are inserts and the second 4 are sends. under inserts choose the MIDI effect named ‘Control’ and it will open automatically with possibility to choose 8 different controls, but that is on the first insert. Later you will understand what i mean with this.
Now, turn the a1 in edit mode to edit it’s parameters. For example if you want to control the Cuttoff frequency on a1 the control number is 74. You can see that with pointing the mouse over the Cuttoff knob on the synth Go back to the MIDI Channel and on ‘Control’ plugin under the first control choose the 74th control or named as “Brightness”. Hide the ‘Control’ window, the MIDI channel, and the synth if you have them open already.
And now:
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it is an option to choose wheter you like to use the midibox_cubase_setup.xml script or to make your own script settings.
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go in the Devices/Device Setup… menu
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click on the Add/Remove tab and find ‘Generic Remote’ device, cilck on it and click on the Add button on the bottom right corner of the window. After that you will see on the left window the ‘Generic Remote’ device added.
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click on it and and then click on the Setup tab. Choose the MIDI ports in/out which you have installed.
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bellow the ports is the status which you need to edit it correctly for having great fun!
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click on the first row under the ‘Control Name’ tab and write ‘Fader 1’.
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under the ‘MIDI Status’ tab choose ‘Controller’.
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under the ‘MIDI Channel’ tab choose 1st channel.
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under the ‘Address’ tab choose the address to which fader you’d like to be ‘Fader 1’.
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under the ‘Max. Value’ tab leave it as it is.
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under the ‘Flags’ tab i am not so sure, you need to test it wheter is ®eceive, (T)ransmit or ®elative.
- in the second same window under the ‘Control Name’ tab write ‘Fader 1’.
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under the ‘Device’ tab choose ‘Midi Mixer’.
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under the ‘Channel/Category’ tab choose the named MIDI channel, by default is ‘MIDI 01’.
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under the Value/Action tab choose ‘Brightness Ins 1’.
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under the ‘Flags’ tab try to configure it by yourself, i don’t know what the mother is that thing.
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on the bottom right corner choose ‘Apply’, turn off the window and try some tweaking, it would work.
If it does Congratulations to my tutorial, if not try to find me and spank my ass till death! 
