Use -S (--stamp) to force capturing at a certain time stamp. The usual time syntax can be used.
When a fixed number of captures is used (-n), manually set captures are added to that number, so e.g. $ vcs -n 3 -S 1m will output four captures.
Highlights are a special type of manual captures. They're put in a separate section of the contact sheet (see Parts of a contact sheet), shown before the standard contact sheet and with a highlighted background.
Use -l (--highlight) followed by a time stamp string to add a highlight. There's no limit of highlights.
Note that unless a single highlight is used they won't be centered so it's usually preferable to match the number of columns (or a multiple).
In manual-only mode no captures will be taken by default, only those specified manually will be used.
Manual-only mode is enabled with -m (--manual). At least a manual capture is required to enable this mode.
Highlights (which are always manual) and extended captures aren't affected by this mode.