Richard Stallman of course was an advocate of free software, and VMS was proprietary, so he would've disapproved of it on that basis alone. (To be clear, I am not discussing here whether he was right or wrong, only the historical facts.)
But I came across this fascinatingly elliptical remark on https://www.tech-insider.org/vms/research/1982/0111.html suggesting that he also had some technical criticisms:
From: Richard M. Stallman <RMS at MIT-AI> To: info-vax at SANDIA The deficiencies of VMS I described two years ago, which are in the file RMS;VMS > on MIT-AI, as far as I know have not been fixed. They add up to a lot of problems with trying to make a multi-process command environment (such as is used on ITS and Twenex) to run on VMS, plus lots of assorted uglinesses and things that can't be done.
A Google search turned up no sign of that file. I would be interested in just what his criticisms were.