I'm sorry, I can't suggest what you might have done. There are no edits to index.php for vBookie, so that page shouldn't have been affected as far as I can tell. I've checked on my own forum that has vBookie installed, and the forums collapse and uncollapse fine. Check it out
here. I can only suppose that you either didn't follow the instructions correctly, or didn't realise that something you were asked to do might conflict with some other hack you've got installed.
Perhaps someone who is more familiar with the forum display page might be able to suggest what the problem may be, but I'm sorry I have no ideas.