Let $\vec{a}$, $\vec{b}$ and $\vec{c}$ be three unit vectors such that $\vec{a} \times (\vec{b} \times \vec{c}) = \frac{\sqrt{3}}{2}(\vec{b}+\vec{c})$. If $\vec{b}$ is not parallel to $\vec{c}$, then the angle between $\vec{a}$ and $\vec{b}$ is:
(1) $\frac{3\pi}{4}$
(2) $\frac{\pi}{2}$
(3) $\frac{2\pi}{3}$
(4) $\frac{5\pi}{6}$