I've been unable to find any solution beyond what was described in the previous post, which is why it wasn't in the post in the first place

Anything further would be pure guesswork.
One such guess is that in some occasions, other plugins incorrectly use the
process_templates_complete plugin hook and thus cause a design flaw in vBulletin to be triggered.
You could also try disabling all other plugins and see if the steps above works for you then.
Fillip