Your simulated vortex is really a radial_gravity.
Vortex field applies a force, not a constant velocity. I think your mass was like 0.03 or something like that. I suspect the physics engine is using singles, in which case really teeny mass any really teeny forces combined with high velocity might not be numerically stable.
If you bump up ship mass, to 1, strength of near 1 behaves better, though you'd have to increase the thrust, etc otherwise you can't move the rocket. Maybe a colocated drag field could also help with things going crazy, though I didn't try that yet. Another thought I had but have not tried is two fields with different region, one with negative strength, so you get a sort of narrow annular ring of force, which prevents things from accelerating really high.