Let $H$ be the set of all vectors of the form $\left[\begin{array}{c} -3t\cr 0\cr t\cr \end{array}\right]$. Find a vector $\vec{v}$ in ${\mathbb R}^3$ such that $H=\mathrm{span}\left\lbrace \vec{v} \right\rbrace$.

$\vec{v}=$ $\left[\Rule{0pt}{3.6em}{0pt}\right.$$\left]\Rule{0pt}{3.6em}{0pt}\right.$.