Let $\vec{a},\vec{b},\vec{c}$ be non-zero vectors such that no two are collinear and $(\vec{a} \times \vec{b}) \times \vec{c}=\frac{1}{3} \cdot |\vec{b}| \cdot |\vec{c}| \cdot \vec{a}$. If $\theta$ is the acute angle between the vectors $\vec{b} \ and \ \vec{c}$, then $sin\theta$ is equal to :