Finding optimal row sequence for a melody on 5-row Bayan Accordion, v2.0
To concatenate vectors, you can do this:

(apply #'concatenate 'vector 
       (mapcar (lambda (x) 
                 (typecase x 
                   (vector x) 
                   (t (vector x))))
               list-of-vectors-or-numbers))


PS. Since this is CR, here are some notes on your code:


Doc string (no more than one, but it can be multi-line!) usually comes before the declarations. Please see Syntactic Interaction of Documentation Strings and Declarations: (defun foo (...) (declare ...) "doc") will define a function foo which has no docstring and return "doc", while (defun foo (...) "doc" (declare ...)) will declare a function with a docstring, returning nil.
(declare ((vector number) rowlist)) should be (declare (type (vector number) rowlist)).
elt should not be used if you know that the object is a vector; use aref instead.
There is no need to bind exithigh and enterhigh in should-flip-block since they are used just once.
declaim is, I think, more "idiomatic" than proclaim in your case.
It is clearer to use 1- in vector-last.