vector-push-extend

(vector-push-extend value vector &optional extension)

Like vector-push, but when the vector is full it grows the backing storage (by at least extension elements, default 1) instead of returning nil, so the push always succeeds and the new index is returned. Any vector with a fill pointer can be grown, whether or not it was created :adjustable (matching common practice; adjustable-array-p reports the flag verbatim). Signals an error when the vector has no fill pointer.