Yeah, I'm pretty sure that's correct. Just whipped this up, using the WGS-84 datum.

  (use-modules (geo vincenty))
  
  (let walk ((p '(0 0 180))
             (i 0))
    (cond ((= i 3)
           (display p)
           (newline))
          (else
            (walk (apply vincenty
                         (list (car p) (cadr p) (+ 90 (caddr p)) 10000000))
                  (+ i 1)))))

Running this yields:

  (0.01777744062090717 0.16837322410251268 179.98234155229127)

Surely the discrepancy is down to spheroid vs sphere, yeah?