Enable advanced customizations.

(add-hook 'vertico-mode-hook #'vertico-multiform-mode)

• Per category:

(documentation-property 'vertico-multiform-categories
                        'variable-documentation)

Alist of categories/regexps and list of settings to turn on per category.
See ‘vertico-multiform-commands’ on details about the settings.  The
category settings have lower precedence than
‘vertico-multiform-commands’.

• Per command:

(documentation-property 'vertico-multiform-commands
                        'variable-documentation)

Alist of commands/regexps and list of settings to turn on per command.
Takes precedence over ‘vertico-multiform-categories’.  A setting
can either be a mode symbol, a function, an inverted mode symbol
or function, or a cons cell of variable name and value.  The key
t can be used to specify catch all/default settings.  The value
of ‘this-command’ is used as key for the lookup.