<For unmanned vehicles, I don't think the radiation belts pose any significant threat. Any deep space probe bound for Jupiter would already be radiation hardened. The radiation levels around Jupiter are much more intense than our belts. While not certain, it wouldn't surprise me if Saturn and the other gas giants have substancial radiation belts of their own.

I was assuming a manned mission, not robotic probe. On this point I agree entirely.

Saturn most definately has radiation belts. Im not sure if they we're measured by Voyager though the Magnetic field was. Saturn's planetary magnetic dipole moment is 600 times larger than Earth's although considerable farther from the sun and it's source of energetic charged particles.

Here is a good little primer on the topic whithin which is burried some info on the Saturnian radiation belts.

http://dosxx.colorado.edu/magneto/magnetosphereshome.html

I agree that VASIMR is a great concept but, I'm dubious of the ability to heat the plasma efficiently and effectively with a microwave array in a spacecraft, reactor or no.