  In the vev insertion approximation (VIA) the spacetime dependent part of the
mass matrix is treated as a perturbation. We calculate the source terms for
baryogenesis expanding both the self-energy and propagator to first order in
mass insertions, which gives the same results as the usual approach of
calculating the self-energy at second order and using zeroth order propagators.
This procedure shows explicitly the equivalence between including the mass in
the free or in the interaction Lagrangian. The VIA source then originates from
the same term in the kinetic equation as the semi-classical source, but at
leading order in the derivative expansion (the expansion in diamond operators).
On top, another type of derivative expansion is done, which we estimate to be
valid for a bubble width larger than the inverse thermal width. This cuts off
the divergence in the VIA source in the limit that the thermal width vanishes.
