Filtering items available in media library - taken from v1 of the WooCommerce Product Vendors extension.

functions.php

/**
 * Only show current vendor's media in the media library
 * @param  array $request Default request arguments
 * @return array          Modified request arguments
 */
add_filter( 'request', 'pv_restrict_media_library', 10, 1 );
function pv_restrict_media_library( $request = array() ) {

    if( ! is_admin() ) {
        return $request;
    }

    $screen = get_current_screen();

    if( in_array( $screen->id, array( 'upload', 'product' ) ) ) {

        // Check if user has vendor access to the dashboard
        if( vendor_access() ) {
            $vendor_id = is_vendor();
            if( ! $vendor_id ) {
                return;
            }

            $vendor_admins = get_vendor_admins( $vendor_id );

            if( ! $vendor_admins ) {
                return;
            }

            $admins = array();
            foreach( $vendor_admins as $admin ) {
                if( ! $admin->ID ) {
                    continue;
                }
                $admins[] = $admin->ID;
            }

            if( 0 == count( $admins ) ) {
                return;
            }

            $request['author__in'] = $admins;
        }
    }

    return $request;
}

/**
 * Only show current vendor's media in the media library modal on the product edit screen
 * @param  array $query Default query arguments
 * @return array        Modified query arguments
 */
add_filter( 'ajax_query_attachments_args', 'pv_restrict_media_library_modal', 10, 1 );
function pv_restrict_media_library_modal( $query = array() ) {
    if( ! is_admin() ) {
        return $query;
    }

    $screen = get_current_screen();

    // Check if user has vendor access to the dashboard
    if( vendor_access() ) {
        $vendor_id = is_vendor();
        if( ! $vendor_id ) {
            return;
        }

        $vendor_admins = get_vendor_admins( $vendor_id );

        if( ! $vendor_admins ) {
            return;
        }

        $admins = array();
        foreach( $vendor_admins as $admin ) {
            if( ! $admin->ID ) {
                continue;
            }
            $admins[] = $admin->ID;
        }

        if( 0 == count( $admins ) ) {
            return;
        }

        $query['author__in'] = $admins;
    }

    return $query;
}