given two vectors, $\displaystyle \vec{u}$ and $\displaystyle \vec{v}$ such that $\displaystyle \vec{u} \cdot \vec{v} = |\vec{u}| |\vec{v}|$ explain what you know about these two vectors.
And, of course, having an angle of "0" between them means the vector point in the same direction- one is a multiple of the other. (We should also point out that $\displaystyle cos(180)= 0$. Two angles that point in opposite directions have dot product 0. Of course, one is still a multiple of the other, just with negative multiplier.)