vspace not working after linebreak

\vspace if used mid paragraph, as here, stores the vertical space in a special place (a vadjust node) which is then inserted after the paragraph has been broken into lines, after the line on which the space is added. in your example the \vspace is added in the last line of the paragraph so the space comes after the paragraph. Compare

\documentclass{article}
\begin{document}
\noindent
Hallo \\
How \\
Are \\
You?

xxxxx

\noindent
Hallo \\
How \\
Are \\
\vspace{15cm}You?

xxxxx
\end{document}

to add a space at the linebreak you should use

\\[3cm]

but it is almost always wrong to use \\ at all apart from its use in tables.