How do you propose to power it?
A Mars-bound VASIMR needs a hefty power-supply to have a decent trip-time else it's a SLOW way to travel, just like an ion-drive. Getting there in the oft quoted "39 days" requires a 200 MWe reactor - gas core with an MHD converter to get the right power/mass ratio - and still needs a re-entry vehicle doing an interplanetary speed aerobrake, while the mother-ship brakes the slow-way into orbit.