From my tests, it seems that the healing from the gray goo vials is a prerequisite for getting the healing from V-Ko, and I can't think of anything else that could be another prerequisite. The button to perform the upgrade is on the V-Ko's (or Vi's) main menu, on one of the light purple-ish directional buttons (same as North for the V-Ko on Mhen'ga, same as West for Vi).