If u_n is in a second order continuous Lagrange space, then the vorticity is in a Discontinuous Lagrange space of order 1.
Thus your projection to CG-1 is introducing an error between the actual and shown vorticity.
Please use a DG-1 space for the projection, and XDMFFile.write_checkpoint to being able to visualize the DG-1 space in Paraview, as compute_vertex_values only does a CG-1 interpolation of any function.