  Vortex states in high-$T_{\rm c}$ superconductors with point defects are
studied by large-scale Monte Carlo simulations of the three-dimensional
frustrated XY model. A critical point is observed on the first-order phase
boundary between the vortex slush and vortex liquid phases. A step-like anomaly
of the specific heat is detected in simulations of finite systems, similar to
an experimental observation [F.~Bouquet {\it et al.}, Nature (London) {\bf
411}, 448 (2001)]. However, it diminishes with increasing system size, and the
number and size distribution of thermally-excited vortex loops show continuous
behaviors around this anomaly. Therefore, the present study suggests a
crossover rather than a thermodynamic phase transition above the critical
point.
