If $ \vec{a} $ and $ \vec{b} $ are unit vectors such that the vector $ \vec{a} + 3\vec{b} $ is perpendicular to $ 7\vec{a} - \vec{b} $ and $ \vec{a} - 4\vec{b} $ is perpendicular to $ 7\vec{a} - 2\vec{b} $, then the angle between $ \vec{a} $ and $ \vec{b} $ is: