  We examine the effective interaction of nonrelativistic fermions with an
external vector field in superfluid systems. In contrast to the complicated
vertex equation, usually used in this case, we apply the approach which does
not employ an explicit form of the pairing interaction. This allows to obtain a
simple analytic expression for the vertex function only in terms of the order
parameter and other macroscopic parameters of the system. We use this effective
vertex to analyze the linear response function of the superfluid medium at
finite temperatures. At the time-like momentum transfer, the imaginary part of
the response function is found to be proportional to the fourth power of small
Fermi velocity, i.e. the energy losses through vector currents are strongly
suppressed. As an application, we calculate the neutrino energy losses through
neutral weak currents caused by the pair recombination in the superfluid
neutron matter at temperatures lower than the critical one for S-wave pairing.
This approach confirms a strong suppression of the neutrino energy losses as
predicted in Ref.[4].
