To meet the high power requirements of the VASIMR engine, an onboard nuclear reactor will be needed to power the craft. Research has already been done on fission power in space, but it has been underfunded in the past. With proper funding and modern equipment, it should be possible to make a space hardened reactor capable of providing more than 1MWe.
The SP-100 was a joint DOD/DOE/NASA program to design a space-application nuclear reactor that operates in the 10-1000kW range. Current systems, such as Alkali Metal Thermal-to-Electric Conversion (AMTEC) can achieve this with greatly reduced mass.
The following are a few images from the initial design of the SP-100
Here is a table of masses of current on-board nuclear reactors:
Looking at the masses of the smaller reactors, we expect that, given a full research budget, it would be possible to develop a suitable reactor with mass 2 tons.