The features of nuclear steam and VASIMR are pretty much a list of opposite pros and cons. E.g.:
nuclear steam doesn't waste any mass with electrical generating components so it is lighter overall than VASIMR.
Contrast this with VASIMR which can run on solar arrays and can share its electrical power source with other components.
Nuclear steam has a lower exhaust velocity so the overall power source requirement is lower.
Vasimir's higher velocity mean the specific impulse of reaction mass is 5x greater than nuclear steam, reducing carried mass and power generating needs. This has significant impact when duration of thrust is very large.
These attributes define the design envelopes.
If you need occassional thrust without a lot of mass and already have an electrical power source, VASIMR is good (e.g. orbital correction for satellites and space stations).
If you want to move a payload under continous thrust for days on end, a nuclear rocket is a good choice.
If you have a payload that has a fairly beefy electrical power source that you want to move under continous thrust for weeks on end, VASIMR is worth considering but may or may not be the best choice.
If you want to move a payload under continous thrust for a many months, go with VASIMR.
E.g. a russian ERTA generator can produce 150kW for 1.5 years while weighing 7500kg. A 150kW VASIMR drive would weigh 225kg and produce 4N. Fuel for 1.5 years is 9300kg. Total starting mass for 1.5years of 4N thrust is 17,025kg.
The SNTP nuclear rocket weighs ~13kg/N so 50kg of motor. Generously assuming the nuclear fuel would last 1.5 years, it still needs 49,000kg of reaction mass. Total starting mass for 1.5 years of 4N thrust is 49,050kg.
Assuming I've done the math right (which is not guaranteed since it involves partial fraction calculus) under that whopping 4N of thrust the VASIMR rocket will crank up to yawn-inspiring 0.004 m/s while the nuclear rocket will do a pokey 0.0012 m/s. Distance wise, the Vasimir will traverse 10.8 km vs. the nuclear rocket has only covered 3.8km.
While utterly theoretical, it does show that for ultra long burns, the reactor overhead of VASIMR is outweighed by the reaction mass increase of a nuclear rocket.