Organic Groups Vocabulary - Dynamically Load Terms
I recently encountered the same issue and the way I got around it was to use a hook_form_alter to use the appropriate taxonomy from the pre-populated selection of og_group_ref via a db_select on the taxonomy_term_data table.

I used entity_reference_prepopulate to set up the link to create group content for the group.  Seems hacky to me and would like to know if others were able to get it to work like the OP had wanted it.  Here is an excerpt of my hook_form_alter.

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  // ... get gid from context
  // ... i used request_uri() to get the uri and parsed out the gid
  // there may be less hacky ways of doing this.
  $query = 'SELECT tdata.tid, tdata.name FROM (SELECT vid from {og_vocab_relation} where gid=:groupid) og left join {taxonomy_term_data} tdata on (og.vid=tdata.vid)';
  $query_params = array(':groupid' =&gt; $gid);
  $taxi_terms = db_query($query, $query_params);
  $options = array();
  $options['_none'] = '- None -';
  foreach($taxi_terms as $taxi_term) {
    $options[$taxi_term-&gt;tid] = $taxi_term-&gt;name;
  }
  $form['og_vocabulary'] =  array(
    '#title' =&gt; 'Projects',
    '#type' =&gt; 'select',
    '#multiple' =&gt; TRUE,
    '#options' =&gt; $options
  );
}