I am upgrading from ProTools 10 to ProTools 12.6, and from VEP 5 to VEP 6. I have two boot volumes, one with PT 10/VEP 5 and one with PT 12.6/VEP 6.

I have a PT session that is coupled to VEP 5; this session opens and plays correctly after launching VEPro Server 64-bit and using the PT10/VEP 5 combo in the volume where it was originally created. When I attempt to open the same session after booting from the second volume with PT 12.6 and VEP 6 and launching VEP 6 Server 64-bit, x, I get an error saying the Vienna Ensemble Pro plugin is not found. If I click OK to dismiss the error and let the session open anyway, I can see the greyed out plugin assignments, all of which reference 'VEPro". I can click on the insert with the greyed out 'VEPro' label and select the instrument option for  'VIenna Ensemble Pro (stereo)' -- so the VEP 6 plugin is, in fact installed and visible to PT. If I do this, what appears in the Insert is labeled 'Vienna Ensemble Pro' and not 'VEPro', I get a AEA error -20007 (which I cannot find in any PT docs or by Googling), and there is no sign of the coupled VEP instance. I can load a new instance, but, as expected, the setup associated with the coupled instance is lost.  Thoughts on how to proceed?

Thanks, DG. You put me on the right track. I uninstalled VEP 6 from the PT 12.6 volume and installed VEP 5 there. That eliminated the 'Plugin not found' error. I then had to redo all the input channel assignments in ProTools to see the slightly differently named inputs. That has gotten back to having a working mix. At some point, I'll try upgrading to VEP 6 and seeing if all remains good.