Suppose \vec{u} = \langle 4, -2, -4 \rangle. Then
\langle 8, 0, 8 \rangle makes an acute angle with \vec{u}
\langle 5, 4, 3 \rangle makes an acute angle with \vec{u}
\langle 3, -2, 5 \rangle makes an acute angle with \vec{u}
\langle 4, 1, -3 \rangle makes an obtuse angle with \vec{u}