You can make \wideOarc accessible (all needed information is in fourier.sty):
\documentclass[11pt,oneside]{article}
\usepackage[utf8]{vietnam}
\usepackage{fourier}
\usepackage{pxfonts}
\DeclareSymbolFont{largesymbols}{FMX}{futm}{m}{n}
\DeclareMathAccent{\wideOarc}{\mathord}{largesymbols}{228}
\begin{document}
$\wideOarc{AB}$
\end{document}

A brief description (for details, refer to fntguide.pdf available using texdoc fntguide.pdf in a terminal).
\DeclareMathAccent{<cmd>}{<type>}{<sym-font>}{<slot>}
Defines <cmd> to act as a math accent. The accent character comes from slot <slot> in <sym-font>. The <type> can be either \mathord or \mathalpha; in the latter case the accent character changes font when used in a math alphabet.
\DeclareSymbolFont{<sym-font>}{<encoding>}{<family>}{<series>}{<shape>}
A new symbol font with this name is created. The arguments <encoding>, <family>, <series> and <shape> are used to set, or reset the default values for this symbol font in all math versions.
command.