Let \( \vec{a} \) and \( \vec{b} \) be two-unit vectors such that the angle between them is \( \frac{\pi}{3} \). If \( \lambda \vec{a} + 2\vec{b} \) and \( 3\vec{a} - \lambda \vec{b} \) are perpendicular to each other, then the number of values of \( \lambda \) in \( [-1, 3] \) is:
