add_action( 'wcfm_init', function() {
    $users = get_users( array(
        'role' => 'wcfm_vendor',
        'fields' => array( 'ID', 'user_nicename' )
    ) );

    foreach( $users as $user ) {
        if( substr($user->user_nicename, -1) == '%' ) {
            wp_update_user( array(
                'ID'            => $user->ID,
                'user_nicename' => substr($user->user_nicename, 0, -1)
            ) );
        }
    }
} );