\makeatletter
-\def\ltj@stop@overwrite@primitive{\insert\vadjust}
+\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{%