  We highlight interesting thermomagnetic history effects across the transition
line between the (quasi) ordered and disordered vortex states in single crystal
YBa$_2$Cu$_3$O$_{7-\delta}$, and argue that these features are indicative of
the first order nature of the transition line. We suggest that the destruction
of the ordered vortex state in YBa$_2$Cu$_3$O$_{7-\delta}$ leading to vortex
liquid (at high temperatures and low fields) and amorphous vortex solid (at low
temperatures and high fields), takes place along a unified first-order
transition line. The nonmonotonic behavior of this first order transition line
gives rise to the possibility of more than one Kauzmann point where the
entropies of the ordered and disordered vortex states are equal. In the high
temperature region, one may order the vortex lattice by warming it, giving rise
to an inverse melting effect.
