  We propose a new parametrization of the four-point vertex function in the
one-loop one-particle irreducible renormalization group (RG) scheme for
fermions. It is based on a decomposition of the effective two-fermion
interaction into fermion bilinears that interact via exchange bosons. The
numerical computation of the RG flow of the boson propagators reproduces the
leading weak coupling instabilities of the two-dimensional Hubbard model at Van
Hove filling, as they were previously obtained by a temperature RG flow.
Instead of regularizing with temperature, we here use a soft frequency
$\Omega$-regularization that likewise does not artificially suppress
ferromagnetism. Besides being more efficient than previous N-patch schemes,
this parametrization also reduces the ambiguities in introducing boson fields.
