Yes, I have had this with an orchestral VSTi, and in my case it wasn't the Midi Volume (CC7) that was causing trouble. It turned out to be a CC number which Cubase thinks should be chased automatically (there are a number of these that are hard wired for chasing in Cubase, CC7 is one of them, Aftertouch is another etc.). I sorted it by looking in the preferences of the VSTi and changing it's default CC number to one which isn't chased. It could have been the Expression CC but I can't 100% remember.
To see which CCs are always chased by Cubase use MidiOX, or maybe you can use the built-in Midi Monitor plugin these days.
This also effects one of my hardware synths too, so if this volume change problem happens for external synths then try using midiox to filter the CC out. Place it between two midiyoke ports.... Couldn't change the CC on the external synth so filtering out was the only way.
There is also another source of problem that can occur, and that's adding midi automation to different lanes when they're on the same instrument and the same midi channel. This causes havoc because now you've got two competing automation lanes controlling the same thing, and on my system it's not always clear which one gets priority. Sometimes to avoid this I prepare a midi track which I use just for automation duties, with automation off on all other midi tracks. Keeps it easier to understand then!