I did already comment on this in a previous thread. chosen is dependent on JQuery or alternatively the Prototype framework.
To get chosen to work with Vanilla JS would involve editing the chosen script — not a good idea.
edit: I also seem to remember from similar issues you had some time ago, that a possible workaround was to simulate clicking on that option. I maybe wrong.
Just a quick search back
In that example I dispatch the change event as well.