  We study the onset of vector instabilities in the post-inflationary epoch of
the Universe as a mechanism for primordial magnetic fields amplification. We
assume the presence of a charged spectator scalar field arbitrarily coupled to
gravity during Inflation in its vacuum de Sitter state. Gravitational particle
creation takes place at the transition from Inflation to the subsequent
Reheating stage and thus the vacuum field state becomes an excited many
particles one. Consequently this state can be described as a real fluid, and we
build out the hydrodynamic framework using second order theories for
relativistic fluids with a relaxation time prescription for the collision
integral. Given the high-temperature regime and the vanishing scalar curvature
of the Universe during Reheating (radiation-dominated-type era), the fluid can
be regarded as a conformal one. The large quantum fluctuations induced by the
rapid transition from inflationary to effectively radiation dominated expansion
become statistical fluctuations whereby both a charge excess and anisotropic
pressures are produced in any finite domain. The precise magnitude of the
effect for each scale is determined by the size of the averaging domain and the
coupling to curvature. We look at domains which are larger than the horizon at
the beginning of Reheating, but much smaller than our own horizon, and show
that in a finite fraction of them the anisotropy and charge excess provide
suitable conditions for a Weibel instability. If moreover the duration of
reheating is shorter than the relaxation time of the fluid, then this
instability can compensate or even overcome the conformal dilution of a
primordial magnetic field. We show that the non-trivial topology of the
magnetic field encoded in its magnetic helicity is also amplified if present.
