I am able to reproduce this.
The parameter only works on first run. It does not appear to affect a running instance, either from a normal shortcut or
Run application '{VA_DIR}\VoiceAttack.exe' -with parameters '-opacity 20'
To clarify, the parameter "-opacity" does not work on a running instance at any time, including when "-toggledark" in active, however, "-toggledark" does work when called using the "Run An Application" action or a normal shortcut.
I'm running Windows 7 Home Premium x64.