Allow vote and comments only for a special group

There is no option for that.

You would have to edit the file "form" of the view "vote" via the menu Display->Views and change the line:
if(($access_vote == 'registered' && empty($hikashop_vote_user_id)) || ($access_vote == 'buyed' && $purchased == 0)){

to something like that:
$my = hikashop_loadUser(true);
$config =& hikashop_config();
$userGroups = JAccess::getGroupsByUser($my->id, (bool)$config->get('inherit_parent_group_access'));
$allowedGroups= array('XXX','YYY');
$inter = array_intersect($userGroups,$allowedGroups);
if(!empty($inter)){

where XXX and YYY are the ids of the user groups allowed access to the voting.