[Paul M]

Your problem is nothing to do with this hack - you have hit a vbulletin limitation.

Your reputation is 2,147,483,647 - this is the maximum value that the database field that stores the reputation value can hold - if you add any more to it, the field overflows and flips to being a very large negative number.

I notice you have 3.5.0 installed - in later versions (3.5.4 ?) a check was added that stops the overflow - instead the value stays at 2,147,483,647.