REST Endpoint API 404

There are few problems:

1. you are using \WP_REST_Server::CREATABLE which is executed only when the request type is POST, if you are just opening the URL in the browser use \WP_REST_Server::READABLE instead (or \WP_REST_Server::ALLMETHODS to accept any request type).

2. your callback is 'callback' => [$this, 'import_csv'], but it should be 'callback' => [$this, 'import_csv_file'], as the import_csv() method is not defined so i am guessing you meant import_csv_file().

3. the return new \WP_REST_Response($data, 200); should be return new \WP_REST_Response($req, 200); as $data variable does not seem to be defined.

The complete code should look like this

class Import_Csv
{
    public function register_routes()
    {
        $version = 1;
        $namespace = sprintf('vendor/v%d', $version);
        $base="/import/";

        \register_rest_route(
            $namespace,
            $base,
            [
                [
                    'methods' => \WP_REST_Server::READABLE,
                    'callback' => [$this, 'import_csv_file'],
                    'permission_callback' => [$this, 'get_import_permissions_check'],
                    'args' => []
                ]
            ]
        );
    }

    public function get_import_permissions_check($req)
    {
        return true;
    }

    public function import_csv_file($req)
    {
        # the import process
        return new \WP_REST_Response($req, 200);
    }
}