  We extend our previous analysis of the motion of vortex lines [I.
Bialynicki-Birula, Z. Bialynicka-Birula and C. Sliwa, Phys. Rev. A 61, 032110
(2000)] from linear to a nonlinear Schroedinger equation with harmonic forces.
We also argue that under certain conditions the influence of the contact
nonlinearity on the motion of vortex lines is negligible. The present analysis
adds new weight to our previous conjecture that the topological features of
vortex dynamics are to a large extent universal.
