You can notice the v in the lambda in the function body, where is the v coming from, what it is based on?
(define (cached-assoc xs n)
(letrec ([memo (make-vector n #f)]
[acc 0]
[f (lambda(x)
(let ([ans (vector-assoc x memo)])
(if ans
(cdr ans)
(let ([new-ans (assoc x xs)])
(begin
(vector-set! memo acc (cons x new-ans))
(set! acc (if (= (+ acc 1)) 0 (+ acc 1)))
new-ans)))))])
(lambda (v) (f v))))
The whole expression is returning a lambda as a result, and in that lambda there's a formal parameter named v. It doesn't have a value yet, you'll need to call the lambda to bind a value to v and produce a result (assuming the code is working):
((letrec ([memo (make-vector n #f)] ; notice the extra opening `(`, we want to call the returned lambda
[acc 0]
[f (lambda(x)
(let ([ans (vector-assoc x memo)])
(if ans
(cdr ans)
(let ([new-ans (assoc x xs)])
(begin
(vector-set! memo acc (cons x new-ans))
(set! acc (if (= (+ acc 1)) 0 (+ acc 1)))
new-ans)))))])
(lambda (v) (f v)))
10) ; <- the value 10 gets bound to `v`
However, your code isn't right. You are referring to variables named n and xs, but they are not defined anywhere and need a value of their own. The procedure vector-assoc doesn't exist. Also, the lambda at the end is redundant, you could simply return f, there's no need to wrap it in an additional lambda. Finally: you should define the whole expression with a name, it'll make it easier to call it.
I won't go into more details because first you need to fix the function and make it work, and is not clear at all what you want to do - but that should be a separate question.