A function to add arguments to a taxonomy

Use the filter register_taxonomy_args to manipulate the arguments used for registering the taxonomy just before it is registered:

function wpse_406824_vendor_taxonomy_args( $args, $name ) {
    if ( $name === 'dc_vendor_shop' ) {
        $args['show_in_rest'] = true;
        $args['query_var']    = true;
        $args['show_ui']      = true;
    }

    return $args;
}

add_filter( 'register_taxonomy_args', 'wpse_406824_vendor_taxonomy_args', 10, 2 );

You’ll need to place this code in a MU plugin or in a custom plugin that’s loaded before the marketplace one (I just checked the source of it and they incorrectly register the taxonomy when the plugin loads, instead of on the init hook – the code above needs to already be loaded in order for it take effect).