(defun vector-neighbor-indices (vector i)
  (let ((result '()))
    (unless (= i 0)
      (push (1- i) result))
    (unless (= i (1- (length vector)))
      (push (1+ i) result))
    result))



CL-USER> (vector-neighbor-indices #(0 1 2 3 4 5 6) 2)
(3 1)

CL-USER> (vector-neighbor-indices #(0 1 2 3 4 5 6) 0)
(1)

CL-USER> (vector-neighbor-indices #(0 1 2 3 4 5 6) 6)
(5)