The gauged sigma model with target $mathbb{P}^1$, defined on a Riemann surface $Sigma$, supports static solutions in which $k_+$ vortices coexist in stable equilibrium with $k_-$ antivortices. Their moduli space is a noncompact complex manifold $M_{(k_+,k_-)}(Sigma)$ of dimension $k_++k_-$ which inherits a natural Kahler metric $g_{L^2}$ governing the models low energy dynamics. This paper presents the first detailed study of $g_{L^2}$, focussing on the geometry close to the boundary divisor $D=partial M_{(k_+,k_-)}(Sigma)$. On $Sigma=S^2$, rigorous estimates of $g_{L^2}$ close to $D$ are obtained which imply that $M_{(1,1)}(S^2)$ has finite volume and is geodesically incomplete. On $Sigma=mathbb{R}^2$, careful numerical analysis and a point-vortex formalism are used to conjecture asymptotic formulae for $g_{L^2}$ in the limits of small and large separation. All these results make use of a localization formula, expressing $g_{L^2}$ in terms of data at the (anti)vortex positions, which is established for general $M_{(k_+,k_-)}(Sigma)$. For arbitrary compact $Sigma$, a natural compactification of the space $M_{(k_+,k_-)}(Sigma)$ is proposed in terms of a certain limit of gauged linear sigma models, leading to formulae for its volume and total scalar curvature. The volume formula agrees with the result established for $Vol(M_{(1,1)}(S^2))$, and allows for a detailed study of the thermodynamics of vortex-antivortex gas mixtures. It is found that the equation of state is independent of the genus of $Sigma$, and that the entropy of mixing is always positive.
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