If
\[
\vec a=\hat i-\hat j+2\hat k,\qquad
\vec b=-\hat i+2\hat j
\]
and \(\vec c\) are three vectors such that \(\vec a+\vec b\) is parallel to \(\vec c\) and
\[
(\vec c+\vec a)\cdot(\vec c+\vec b)=7,
\]
then the vector \(\vec c\) having minimum length is