Hm, the tricky part I think is if you want to specifically control the output of Visual Cortex, since Cortex kind of expects to be your final output encoder. With a pre-encoder expander you could take the output you like from Cortex and patch it elsewhere, re-encoding it later with a separate output encoder like Cadet II. Within VC basically you have the VC input over the channel B inputs, which can act as an opacity control in the center position, since it acts as a full color crossfader between channels A and B.
Are you working with a component or composite camera input? If it’s composite, or you only need the luma signal from the camera, you may be able to get the effect you want by patching the luma from the input decoder through a fader (Bridge, Cadet VI) first, and controlling the fader with your envelope from Sensory Translator. Controlling full-color opacity in an analog system requires substantially more involved circuitry than gating a single signal path, but is achievable using either the VC compositor or an advanced compositor like Marble Index.
Are your Animator & Key Generator minipots fully CCW? These set rise and fall times for the animation voltage. If they are fully CCW, the animation should respond more or less immediately to triggers; otherwise the transition will lag going each direction depending on the minipot settings. I think of the button / trigger input more like a “take” button on a video production mixer: you trigger it to do a wipe with constant slope. When using the slider these minipots act as slew controls for the slider motion.