Mix = 0 = CPU = 0

When mix set to Zero there should be no CPU usage by a VST or VSTi plugin; a value set to 0 should equal to a bypassed plugin. The reason is the fact that automating Bypass function (on/off) can cause click noise depending on plugin wheras automating mix (wet/dry) does not.

Not sure whether you understand what I mean, I would like to be able to automate effect mix knob (dry/wet) and when set to "0" it should equal a bypassed plugin = no CPU usage.

Well, it might actually work if Reaper also did a fade in for the plugin's audio input once the wet/dry parameter was raised from zero. OTOH this would cause problems for people who are not interested in the 0% wet position not consuming CPU. It all isn't as simple as it seems at first thought.