[racket] how can I change only one element in the vector?

"make-vector" is creating a new vector populated with *references* to a 
single #("1" "2" "3") vector, *not* *copies* of the #("1" "2" "3") vector.

Since you've created one #("1" "2" "3") vector, changing it through one 
reference changes all references to it.

(Aside to PLT people: This might be an example of having the default 
writer show sharing could actually be helpful to beginners.)

-- 
http://www.neilvandyke.org/