How to debug cron jobs

I am trying to debug a cron job in the plugin disqus-comment-system.

$ts = time() + 300;
wp_schedule_single_event($ts, 'dsq_sync_forum');

add_action('dsq_sync_forum', 'dsq_sync_forum');

function dsq_sync_forum($last_comment_id=false, $force=false) {

.........some code

}

I am using vim and vdebug. I tried many times but its doesnot debug the function dsq_sync_forum.

I googled regarding this, but not understood the solutions.