The statistical mechanics of nearly parallel vortex filaments confined in the unbounded plane by angular momentum, first studied by Lions and Majda (2000), is investigated using a mean-field approximation to interaction and a spherical constraint to develop an explicit formula for the mean square vortex position or length scale of the system, R, verified with Path Integral Monte Carlo simulations. In addition we study the previously unstudied microcanonical distribution for this system with applications to plasmas. We confirm that 3D filaments resist confinement in a different way than 2D point vortices and that this results in a profound shift at high-densities for the length scale of quasi-2D versus strictly-2D models of vorticity fields in which angular momentum is conserved. Our analytical results correspond well with those of the Monte Carlo simulations and show a 3D effects contributing significantly to determination of the length scale and to transitions to turbulence at high temperature. We also find negative specific heat in the case of the electron magnetohydrodynamic (EMH) plasma model, a rare phenomenon found in gravo-thermal collapse of globular clusters. This find has implications for magnetic nuclear fusion where compression is crucial to a self-sustaining nuclear reaction.