s is a hot function, so there are performance impacts.
  (assert-vlist vlist)
  (block-cons item vlist #f))

(define (vlist-head vlist)
  "Return the head of VLIST."
  (assert-vlist vlist)
  (let ((base   (vlist-base vlist))
        (offset (vlist-offset vlist)))
    (block-ref (block-content base) offset)))

(define (vlist-tail vlist)
  "Return the tail of VLIST."
  (assert-vlist vlist)
  (let ((base   (vlist-base vlist))
        (offset (vlist-offset vlist)))
    (if (> offset 0)
        (make-vlist base (- offset 1))
        (make-vlist (block-base base)
                    (block-offset base)))))

(define (vlist-null? vlist)
  "Return true if VLIST is empty."
  (assert-vlist vlist)
  (let ((base (vlist-base vlist)))
    (and (not (block-base base))
         (= 0 (block-size base)))))