  We present a functional renormalization group calculation for the
two-dimensional (t,t')-Hubbard model at Van Hove filling. Using a channel
decomposition we describe the momentum and frequency dependence of the vertex
function in the normal phase. Compared to previous studies that neglect
frequency dependences we find higher pseudo-critical scales and a smaller
region of d-wave superconductivity. A large contribution to the effective
interaction is given by a forward scattering process with finite frequency
exchange. We test different frequency parameterizations and in a second
calculation include the frequency dependence of the imaginary self-energy. We
also generalize the channel decomposition to frequency-dependent fermion-boson
vertex functions.
