By mix envelope, I mean the ability to assign an envelope that controls the mix of the various (in the case of Rapture) elements, with this envelope being part of the patch. As far as I am aware, the Rapture (or Rapture Pro) vector mixer cannot be controlled by an envelope within the instrument. It CAN be controlled in realtime, or by automation in the sequencer (e.g. Sonar). But not by an envelope that is part of the patch. In my case, this envelope typically loops infinitely.
In contrast, a 4-dimensional mix envelope is a fundamental feature of the Wavestation (both HW and SW), Virsyn Cube 2, and Camel Audio Alchemy and can be argued as a feature that fundamentally defines the unique sound of each. It is a feature that I frequently use to make a PATCH sound dynamic.