That's because the vsti parameter needs it's own automation data. If you put your automation into "write" mode, you will be able to record the data that is controlling the effect.

Automation and MIDI CC control are two different things in Reaper. Automation is captured as an envelope of control values on the track. The envelope values are used to control one of the VST(i)'s parameters (using a high resolution, I think). A MIDI Controller knob or slider can be assigned to the envelope, using MIDI Learn (the "lrn" button on the envelope. You can tie the envelope points to clips so that they move when you move the MIDI clip (it's in the [Options] menu) and you can select, copy and paste envelope points (right-click and drag through the points to select them, Ctrl+c and Ctrl+v) to another location (even to another envelope) To record automation, set the track's Automation mode to "Write", press Play and adjust the slider. To use the automation set the track's Automation mode to "Read", press Play MIDI CC automation can be used when the VST(i) supports it. The MIDI CC messages are recorded (or entered) within a MIDI clip, just like MIDI notes). The MIDI CC messages have a resolution of 0-127. Sorry, automation envelopes cannot be converted to MIDI CC messages or vice versa. But there some related Feature Requests: HTH

I stand corrected. The last time I looked there for things was in 2.1 or something. :)

Ok, I feel like I understand everything now. Much appreciated. How would I go about finding the right cc parameters for some of my plugins that don't have clear number lists? Some vstis I have work perfectly, Hypersonic2 for example has a great midi learn function that lets you clear the cc number and assign your own but some plugins, especially fx ones don't have midi learn or a list of parameter numbers.

Glad you're sorted now. Developers can use any MIDI CC numbers that they want for any control. So you'll probably need to contact the developer or look round their website.

Many plugins have a "MIDI implementation chart" appended to the manual or in a readme file. If they don't, that information can be found on the website they come from sometimes. If they all fail only experimenting will help, meaning assigning CC to controller knob and checking which control on the plugin GUI moves.