\vec{x} but with arrow from right to left?

A \cev command that seems to give good results on most letters and works correctly (apart a very small drift) in subscripts and superscripts.

\documentclass{article}
\usepackage{graphicx,accents}

\makeatletter
\DeclareRobustCommand{\cev}[1]{%
  \mathpalette\do@cev{#1}%
}
\newcommand{\do@cev}[2]{%
  \fix@cev{#1}{+}%
  \reflectbox{$\m@th#1\vec{\reflectbox{$\fix@cev{#1}{-}\m@th#1#2\fix@cev{#1}{+}$}}$}%
  \fix@cev{#1}{-}%
}
\newcommand{\fix@cev}[2]{%
  \ifx#1\displaystyle
    \mkern#23mu
  \else
    \ifx#1\textstyle
      \mkern#23mu
    \else
      \ifx#1\scriptstyle
        \mkern#22mu
      \else
        \mkern#22mu
      \fi
    \fi
  \fi
}

\makeatother

\begin{document}
$Xx$

$X\cev{x}_{\cev{x}_{\cev{x}}}$ $\cev{a}\cev{b}\cev{m}\cev{X}$

$X\vec{x}_{\vec{x}_{\vec{x}}}$ $\vec{a}\vec{b}\vec{m}\vec{X}$

$\cev{\imath}$

$\vec{\imath}$
\end{document}

A different implementation:

\documentclass{article}
\usepackage{graphicx}

\makeatletter
\DeclareRobustCommand{\cev}[1]{%
  {\mathpalette\do@cev{#1}}%
}
\newcommand{\do@cev}[2]{%
  \vbox{\offinterlineskip
    \sbox\z@{$\m@th#1 x$}%
    \ialign{##\cr
      \hidewidth\reflectbox{$\m@th#1\vec{}\mkern4mu$}\hidewidth\cr
      \noalign{\kern-\ht\z@}
      $\m@th#1#2$\cr
    }%
  }%
}
\makeatother

\begin{document}
$Xx$

$X\cev{x}_{\cev{x}_{\cev{x}}}$ $\cev{a}\cev{b}\cev{m}\cev{X}$

$X\vec{x}_{\vec{x}_{\vec{x}}}$ $\vec{a}\vec{b}\vec{m}\vec{X}$

$\cev{\imath}$

$\vec{\imath}$

$\cev{\sigma}\cev{x}$
$\vec{\sigma}\vec{x}$
\end{document}

This may not be the best way, but two \reflectbox commands will do the trick:

\documentclass{article}
\usepackage{graphicx}
\newcommand{\cev}[1]{\reflectbox{\ensuremath{\vec{\reflectbox{\ensuremath{#1}}}}}}
\begin{document}
\[
\vec{a} \quad \cev{a} \quad \vec{b} \quad \cev{b}
\]
\end{document}

The extensible \overleftarrow might do the trick. The problem is that the arrow is bigger than the one used for \vec, as shown by \[\overleftarrow{a}\overrightarrow{a}\vec{a}\] . To have consistent arrows, you should redefine $\vec$ :

\documentclass{minimal}
\renewcommand\vec[1]{\overrightarrow{#1}}
\newcommand\cev[1]{\overleftarrow{#1}}

\begin{document}
\[\cev{a}\vec{a}\]
\end{document}

Edited to add:

Another solution is the \overset command from amsmath, used with \leftarrow. However, the arrow is still bigger than the one from the \vec command.

\documentclass{minimal}
\usepackage{amsmath}
\begin{document}
\[\overset{\leftarrow}{v}\overset{\rightarrow}{v}\vec{v}\]
\end{document}

*Edited to add * Using \shortleftarrow from the stmaryrd package slightly improves the above construction.