When you enable, or dissable commenting in drupal, you will only affect new nodes, there is no bulk action for this in core (but it should be in my opinion, not every damn useful functionality does have to be in separate module). There are few ways to do it, some proper way would be to built action and then run VBO on nodes, but this is the most time consuming for sure. There are few simpler, most used one is probably just the
update node set comment = 2 where type = "page";this work for drupal 6, for drupal 7 you need to make this twice
UPDATE node SET comment=2 WHERE type='page';
UPDATE node_revision SET comment=2 WHERE nid IN (SELECT nid FROM node WHERE node.type='page');which you run in your phpmyadmin or over ssh with your database. 0 is for disabling, 1 for enabling read and 2 for enabaling read/write
Othere are action writing http://drupal.org/node/172152
Or a module for d6 http://drupal.org/project/mass_change
In drupal 7, actions are simpler so this should work, you create action like thise and then run VBO
<?php
$entity->comment = 2; //replace $entity with whatever VBO says the row represents.
node_save($entity);
?>