Simplified Common Lisp reference
Symbol class: Sequences (Lists, Strings) and Arrays
Symbol type: function
vector-pushnew-elementvector => index or NIL
Argument description:
new-element a object
vector a vector with fill pointer

VECTOR-PUSH function pushes new-element into specified vector. Supplied vector must have fill-pointer (see MAKE-ARRAY). New element is placed at last fill-pointer position and fill-pointer is incremented. See also MAKE-ARRAY, VECTOR-POP and VECTOR-PUSH. Return value is index at which the new item was placed, or NIL if there is no room.

(defparameter *v* (make-array 2 :fill-pointer 0)) => *V*
(vector-push 4 *v*) => 0
(vector-push 3 *v*) => 1
(vector-push 2 *v*) => NIL
*v* => #(4 3)
(vector-pop *v*) => 3
(vector-pop *v*) => 4
Function indexFull documentation for vector-push (HyperSpec)