I think you've missed my point
When you enter a feed url into the vB RSS feed manager you are telling vB to show the RSS feed, via threads in a certain forum.
All this hack does is read the urls from within the vB RSS feed manager and list them on its own list.
If you don't want the vB RSS feed manager to show the feed in threads then you need to untick the feeds within the manager. This will not stop the feeds from showing in the hacks list.
However, the list of feeds in the list on the hack page are just that, a list of URLs to the feeds. So when you double click a feed, or click the button after highlighting a feed all you are doing is telling the hack to go and retrieve the feeds data and display it. If at this point the hack web page can't connect to the server the feed is based on then you will get the error message.
I hope that helps clear the situation up.
If it is something that is happening everytime then there might be something wrong with the URL of the feed .. OR .. perhaps it's a RSS version that the hack can't understand .. not sure to be honest as I didn't write the hack