How to add settings subpage from a plugin to a settings page created in theme?

This is just a typo

Wrong

//wvdh-startpoint
add_submenu_page( 'wvdh-startpoint', 'page-from-plugin', 'menu-from-plugin', 'manage_options', 'plugin-slug', 'wvdh_settings_page' );

Correct

//wdvh-startpoint
add_submenu_page( 'wdvh-startpoint', 'page-from-plugin', 'menu-from-plugin', 'manage_options', 'plugin-slug', 'wvdh_settings_page' );