VASIMR is being tested now
They're testing VASIMR systems in vacuum chambers now. I attended an AIAA talk concerning VASIMR where it was shown this propulsion system has amazing capabilities. What's particularly neat is it can be throttled, i.e. larger thrust at the expense of Isp or microthrust with ridiculous Isp. VASIMR needs a high power nuclear reactor to drive it but that was a given.
Gary