If part: Let \(\vec{a},\vec{b},\vec{c}\) are coplanar
\(\Rightarrow\) Scalar triple product of \(\vec{a},\vec{b}\) and \(\vec{c}\) is zero
[By property of scalar triple product]
Hence \(\vec{a}+\vec{b},\,\vec{b}+\vec{c}\) and \(\vec{c}+\vec{a}\) are coplanar
Only if part: \(\vec{a}+\vec{b},\,\vec{b}+\vec{c},\) \(\vec{c}+\vec{a}\) are coplanar.
Hence, \(\vec{a},\vec{b},\vec{c}\) are coplanar.