  We study the effect of critical fluctuations on the $(B,T)$ phase diagram in
extreme type-II superconductors in zero and finite magnetic field using
large-scale Monte Carlo simulations on the Ginzburg-Landau model in a frozen
gauge approximation. We show that a vortex-loop unbinding gives a correct
picture of the zero field superconducting-normal transition even in the
presence of amplitude fluctuations, which are far from being critical at $T_c$.
We extract critical exponents of the dual model by studying the topological
excitations of the original model. From the vortex-loop distribution function
we extract the anomalous dimension of the dual field $\eta \simeq -0.18$, and
conclude that the charged Ginzburg-Landau model and the neutral 3DXY model
belong to different universality classes. We find are two distinct scaling
regimes for the vortex-line lattice melting line: a high-field scaling regime
and a distinct low-field 3DXY critical scaling regime. We also find indications
of an abrupt change in the connectivity of the vortex-tangle in the vortex
liquid along a line $T_L \geq T_M$. This is the finite field counter-part of
the zero-field vortex-loop blowout. Which at low enough fields appears to
coincide with $T_M$. Here, a description of the vortex system only in terms of
field induced vortex lines is inadequate at and above the VLL melting
temperature.
