vBCredits and many other mods do not support showpost.php in vBulletin 3. This is a feature that's been deprecated in vBulletin 4.0 and above, and due to the fact that it's not a heavily used feature I doubt development time will be allocated to adding the support.
I'll flag this as Not A Bug since it's not a bug in the software, it's simply a lack of support for a specific feature of a specific vBulletin version