If vectors $\vec{a} = 3\hat{i} - 2\hat{j} + 4\hat{k}$ and $\vec{b} = \lambda\hat{i} + \hat{j}$ are perpendicular to each other, then the value of $\lambda$ is : (A) $3$ (B) $-3$ (C) $\frac{2}{3}$ (D) $-\frac{2}{3}$