VASIMR ended up never flying to the ISS after NASA determined that the space station “was not an ideal demonstration platform for the desired performance level of the engines,”
Correct. NASA reversed it's original determination that VASIMR should be tested on IIS. I don't know NASA's rationale for the change. NASA continues to provide funding to Ad Astra to help support continued refinement of VASIMR and the press release from Aug. 2017 details what seems to be very substantial progress.
PRESS RELEASE 080917, AUGUST 9, 2017
AD ASTRA ROCKET COMPANY SUCCESSFULLY COMPLETES ALL NASA NEXTSTEP CONTRACT MILESTONES FOR YEAR TWO, RECEIVES NASA APPROVAL TO PROCEED WITH YEAR THREE.