Okay, if the Cached Post Lifespan was set to 0 before, when you weren't having problems, but now that you changed it to a higher number you are having problems, this is why:
VaultWiki didn't need to update the links before because the post always updated the link on every page load. Now, the link is saved with a cached version of the post and needs to be updated. While VaultWiki is programmed to do this, something is preventing this from occurring.
The only thing I can think of is that another modification is interfering with the process by setting the pagetext field to null when VaultWiki tries to read it, is destroying the $vault object, or preventing it from initializing.
Please post or PM a list of your currently installed modifications.