\input{umtest-preamble}
\usepackage{unicode-math}
\setmathfont{Cambria Math}
\begin{document}
\[ \vec a \quad \vec{a} \quad \vec{abc} \quad \overrightarrow{abc} \quad \underrightarrow{abc} \]
\unimathsetup{growing-accents}
\[ \vec a \quad \vec{a} \quad \vec{abc} \quad \overrightarrow{abc} \quad \underrightarrow{abc} \]
\end{document}