A possible workaround (not real eloquent) would be to create an Autohotkey script to assign a hotkey to do a mouseclick on the increment and decrement buttons.  I tested this script for 3 synths.  I'll probably add others and use this myself in the future.  The script sets in the tray while it's running.  It redefines up and down keys and maps them to click on mouse coordinates.  Unfortunately, each synth has its own location for the increment/decrement buttons.  The particular coordinates and the VST synth name would have to be edited for it to work on other computers.  The coordinates would be different for other screen resolutions.  (Use the Autohotkey utility "Window Spy" to get the coordinates.)  To work, the synth has to be the active window.
 
#SingleInstance force
SetTitleMatchMode, 2
CoordMode, Mouse, Window

#IfWinActive, Rapture Pro
up::
Mouseclick,left,916,89
return

down::
Mouseclick,left,900,89
return
#IfWinActive

#IfWinActive, Z3TA+ 2
up::
Mouseclick,left,830,94
return

down::
Mouseclick,left,814,94
return
#IfWinActive

#IfWinActive, Hybrid
up::
Mouseclick,left,740,75
return

down::
Mouseclick,left,714,75
return
#IfWinActive

return

Rapture Pro has MIDI learn for stepping through programs. Some plug-ins do not manage presets relying on the Host for this function. Pretty sure the VST specs do not cover preset management at all.