  Anisotropic stress perturbations induced by primordial Gaussian vector fields
create non-Gaussianity in curvature perturbations. We found that such
non-Gaussianity closely resembles the local-type non-Gaussianity parametrized
by $f_{\rm NL}$, and generates scale-dependent bias in large-scale structures.
We also found a simple relationship between the scale-dependent bias and the
power spectrum of the vector fields. When the vector fields are interpreted as
primordial magnetic fields, the effective $f_{\rm NL}$ is shown to be always
negative. The scale-dependent bias provides a new approach to probing
primordial vector fields.
