Sectioning commands carry their own fonts and the space above and below them, and an explicit \vspace keeps its size; only the lines between them re-break.
Sections and vertical spacing
This heading, and the space it opens beneath itself, are typeset by TEX: a LA TEX sectioning command carries its own font, size, and the vertical skips above and below it, and Reflow TeX keeps them.
The gap above this sentence is an explicit \vspace{1.5cm}. It keeps its size, while the paragraphs around it still reflow to the width of the column.
A subsection
Subsection headings are smaller and have less space around them. Both come from the document class.
\section*{Sections and vertical spacing}This heading, and the space it opens beneath itself, are typeset by \TeX{}: a
\LaTeX{} sectioning command carries its own font, size, and the vertical skips
above and below it, and Reflow TeX keeps them.
\vspace{1.5cm}The gap above this sentence is an explicit \verb|\vspace{1.5cm}|. It keeps its
size, while the paragraphs around it still reflow to the width of the column.
\subsection*{A subsection}Subsection headings are smaller and have less space around them. Both come
from the document class.