Typesetting the vertical line tone marking used in Pe̍h-ōe-jī

You can stack a rule with appropriate dimensions over the character:

\documentclass{article}

\newcommand{\vla}{}% for safety
\DeclareRobustCommand{\vla}[1]{%
  \leavevmode\vbox{%
    \offinterlineskip
    \ialign{##\cr
      \hidewidth\rule{0.08ex}{0.45ex}\hidewidth\cr
      \noalign{\vskip0.2ex}
      #1\cr
    }%
  }%
}

\begin{document}

P\vla{e}h-\={o}e-j\={\i}

\end{document}

\documentclass{article}
\usepackage{lmodern}
\usepackage{stackengine}
\newcommand\myaccent[1]{%
  \stackengine{-.45ex}{#1}{\scriptsize\textquotesingle}{O}{c}{F}{T}{S}}
\begin{document}
P\myaccent{e}h-\={o}e-j\={i}

P\myaccent{E}H-\={O}E-J\={I}
\end{document}