How to add script to ?
You should concatenate $vars['head'] no replacing it:
Your code should be:

function mytheme_preprocess_page(&amp;$vars, $hook) {
    drupal_set_html_head('&lt;script type="text/javascript" src="http://use.typekit.com/12345.js"&gt;&lt;/script&gt;');
    $vars['head'] .= drupal_get_html_head();
}


Notice the .= after $vars['head'].