\documentclass{article}

\begin{document}

Some text.

% ...in vertical mode
\vspace{2em}

\addvspace{1em}% \addvspace after \vspace

Some text.

\newpage

Some text.

% ...in vertical mode
\addvspace{1em}% \addvspace before \vspace

\vspace{2em}

Some text.

\end{document}