Q:

how to fix the position of a text in page when vspace is applied to other parts

I have an issue with using vspace. I have a page, blank with the following lines
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{epigraph}
\begin{document}
\emph{Alpha Beta}
    \epigraph{\emph{An Alpha is a Beta when Gamma is dropped :D}}{\textit{a donkey}}
\end{document}

I want to put \emph{Alpha Beta} exactly in the center of the page, and \epigraph{\emph{An Alpha is a Beta when Gamma is dropped :D}}{\textit{a donkey}}  exactly at the bottom right of the page. I tried pushing the epigraph down with vspace, it works but the vspace moves \emph{Alpha Beta} aways from the center (pushes it up).

A:

Here are two solutions.  One uses \smash to hide the size of the text at the bottom.  The other uses tikzpagenodes to place things using absolute coordinate.  Note, it take two runs to work.
Because epigraph behaves badly, I put it into a minipage.  The [b] option prevents the text from dropping below the bottom of the page.
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{epigraph}
\usepackage{tikzpagenodes}
\usepackage{showframe}
\begin{document}
\null\vfill
\noindent\emph{Alpha Beta}
\vfill
\noindent\smash{\begin{minipage}[b]{\textwidth}
  \epigraph{\emph{An Alpha is a Beta when Gamma is dropped :D}}{\textit{a donkey}}
\end{minipage}}
\newpage\null
\begin{tikzpicture}[remember picture, overlay]
  \node[right,inner sep=0pt] at (current page text area.west) {\emph{Alpha Beta}};
  \node[above right, inner sep=0pt] at (current page text area.south west)
    {\begin{minipage}[b]{\textwidth}
      \epigraph{\emph{An Alpha is a Beta when Gamma is dropped :D}}{\textit{a donkey}}
    \end{minipage}};
\end{tikzpicture}

\end{document}

