What? What are you talking about? If you need these "bytes" as "chars" then do new String -> toCharArray There is no reason you cannot do that, unless those "bytes" do not represent characters (i.e. a String) and so "converting" them to chars would be pointless anyway.
I still have no idea what you mean by "Her teststr is Vector". Are you saying that this "byte buffer" is a serialised Vector object maybe? In which case this "conversion" is less than pointless anyway.