\documentclass{minimal}
\usepackage{unicode-math}
\begin{document}
\setmathfont{Cambria Math}
\def\vec{\Umathaccent fixed 0 4 "20D7 }
$\vec x \vec M \vec 0 \vec +$ \par
\setmathfont{XITS Math}
\def\vec{\Umathaccent fixed 0 4 "20D7 }
$\vec x \vec M \vec 0 \vec +$
\end{document}
% Local Variables:
% TeX-engine: luatex
% End:
