How do I change taxonomy page callback for a specific vocabulary only?
Since the callback is getting the taxonomy term object as argument, you can check the value in $term-&gt;vocabulary_machine_name to get the machine name of the vocabulary, and see if it is the one you are interested in.

function MYMODULE_taxonomy_page_callback($term) { 
  if ($term-&gt;vocabulary_machine_name == 'MYVOCABULARY') {
     drupal_goto('somewhere'); 
  }
  else {
    return taxonomy_term_page($term);
  }
}