\makeatletter
\def\ltj@stop@overwrite@primitive{\insert\vadjust\/\unhbox\vcenter\piyo}
\makeatother

\documentclass[landscape]{article}
\usepackage[margin=10mm,centering]{geometry}

\usepackage{luatexja}

\def\vcenter{aaa}
\makeatletter
\ltj@overwrite@primitive{\/\vcenter\hoge\moveleft}

\usepackage{breqn}

\expandafter\expandafter\expandafter\ltj@overwrite@primitive\expandafter{\ltj@stop@overwrite@primitive}

\begin{document}
\ttfamily\parindent0pt\footnotesize
\def\chk#1{%
  \par\medskip\leavevmode
  \hbox to 11em{\expandafter\string\csname #1\endcsname:\hss}\ 
    \expandafter\meaning\csname #1\endcsname\\
  \hbox to 11em{\expandafter\string\csname ltj@@#1\endcsname:\hss}\ 
    \expandafter\meaning\csname ltj@@#1\endcsname\\
  \hbox to 11em{\expandafter\string\csname ltj@@orig@#1\endcsname:\hss}\ 
    \expandafter\meaning\csname ltj@@orig@#1\endcsname
}

a\vadjust{\hrule height 1pt width 30pt}bc\\d

\chk{/}
\chk{insert}
\chk{vadjust}
\chk{raise}
\chk{lower}
\chk{moveleft}
\chk{moveright}
\chk{vsplit}
\chk{vcenter}
\chk{unhbox}
\chk{unvbox}
\chk{unhcopy}
\chk{unvcopy}
\chk{lastbox}
\end{document}



