...up to the task of propelling vehicles to Mars in 39 days or so.

The paper that time-frame is quoted from is this one...

Andrew Petro's Presentation from 2002

...but it's a pretty hefty vehicle needed to achieve that performance, some 600 tons of which 476 tons is propellant and 22 tons of payload. Powering a 200 MW VASIMR is no small exercise either requiring considerable advancement in nuclear power in-space. Franklin Chang-Diaz, chief scientist working on VASIMR, discussed the kind of power-source needed in this paper...

Fast, Power-Rich Space Transportation, Key to Human Space Exploration and Survival

...which describes a vapour or plasma core nuclear reactor with an MHD power-extractor/converter which gets a specific power of about 2 kWe/kg of power system. It'd mass about 100 tons to give a 200 MWe supply. Thus the 39 day VASIMR mass-breakdown is...

476 tons propellant
100 tons power-supply
24 tons payload/structure

...quite a hefty machine, but that's the price of top performance.