Sorry to res an old thread, but I am also having this issue.

Scheduled tasks won't run on their own. I keep having to delete the vb_session periodically.
The footer is intact.

I have tried adding the PHP code that you mentioned to test disabling plugins, but it failed due to the fact that I'm using vB Advanced (I didn't consider that =_=).

This issue has been going on for some time now. I really need to find how to fix this.
It was just.... random.

It's normal for the vb_session table to contain data, are you sure it's not removing the appropriate records from that table?

I know that it is, but it's filling with over 80,000 lines which is overloading the file (hence the vb_session if full error messages), and causing a white screen of death on my website.
I have no choice but to empty it in order for my website to function, and it seems like it may be because my scheduled tasks refuse to run on their own.

Is your "Session Timeout" setting set to something really high? You can find this on the "Cookies and HTTP Header Options" settings page.

If it's set to something normal such as 900 and you are not getting a significant number of visitors on your forum (DDoS attack could cause it as well) then there may indeed be something else going on.