;; What I currently have is a hash-table like this one:
(define table (make-hash-table 3))

(hash-set! table 'a '(1 2 3))
(hash-set! table 'b '(4))
(hash-set! table 'c '(5 6))

;; And I can easily fold through unique keys like this:
(hash-fold (lambda (key entries res)
             (cons (cons key (apply + entries)) res))
           '()
           table) ; => ((c . 11) (b . 4) (a . 6))

;; What you suggest is a vhash like this:
(define vhash
  (vhash-cons
   'a 1
   (vhash-cons
    'a 2
    (vhash-cons
     'a 3
     (vhash-cons
      'b 4
      (vhash-cons
       'c 5
       (vhash-cons
        'c 6 vlist-null)))))))

;; But how can I fold through unique keys there?