Let $\vec{a}$ and $\vec{b}$ be the vectors along the diagonal of a parallelogram having area $2 \sqrt{2}$. Let the angle between $\vec{a}$ and $\vec{b}$ be acute. $|\vec{a}|=1$ and $|\vec{a} \cdot \vec{b}|=|\vec{a} \times \vec{b}| \cdot$ If $\vec{c}=2 \sqrt{2}(\vec{a} \times \vec{b})-2 \vec{b}$, then an angle between $\vec{b}$ and $\vec{c}$ is :