If $\vec{a}$, $\vec{b}$, $\vec{c}$ are three vectors with magnitudes $\sqrt{3}$, $1$, $2$ respectively, such that $\vec{a} \times (\vec{a} \times \vec{c}) + 3\vec{b} = \vec{0}$, if $\theta$ is the angle between $\vec{a}$ and $\vec{c}$, then $\sec^2 \theta$ is