I don’t know much about the YITH Multi Vendor plugin. ACF works by adding field groups to existing editors for posts, terms, users and it’s own options pages. It cannot automatically add field groups to the admin pages of other plugins that do not use one of these things.
Like you see, you needed to alter the admin page to display the field. It might be possible to use acf_form() https://www.advancedcustomfields.com/resources/acf_form/ to do what you want. You would need to edit all of the plugin’s admin pages and add an acf form to them. You would set the
form option to false and insert the form inside the existing form on the page.
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