Dear Sandeep,
Please take a look at hemi.usr in the nekexamples repo.
There is a routine interp_v() that returns the interpolated
velocity field at a user-prescribed set of points. You could
easily modify this routine to pass in vorticity and interpolate
that (or any other scalar or vector field).
Please note that you should not duplicate interrogation points
on multiple processors (i.e., each processor should have a
unique set of points, xyz(); one possible configuration is that
MPI rank 0 has a list of points of length n and the other ranks
have a list of length 0; this approach is valid if the length of the
list is not too large).
hth,
Paul