Issue: [size] code before [h4] code, deactivates the header function.

font code has the same effect.

Cannot reproduce this either. In articles, the headline becomes un-editable, but parses correctly... at least using the code fragment you provided.

Please try to reproduce all the issues you are having with headlines (including the other 2 reports you posted) in the following article: http://www.vaultwiki.org/demo/H1-Nest-Breaking

Im trying to reproduce here:
http://www.vaultwiki.org/demo/headli...st?redirect=no
But keep encountering the 'The section you are trying to edit could not be found in the current page.' error.

Only get this to happen if:
[size=2][h=4]Some text[/size][/h]
This is malformed code and vBulletin's parser is likely to just spit it back out as entered.

Yeah, but unfortunately it does get mixed up like this. Im not sure if my users have done this, or if the importer did.
Unleashing large amounts of users on Vaultwiki will get code mixed up. The issue was not present in NuWiki. Though that didnt have chapter editing.

vBulletin's WYSIWYG editor does this. They have tried to "fix" it with some workarounds in the parser, but these workarounds tend to break our headline edit links (and have been giving me a headache all day). Sometimes they help the nesting problem, sometimes they make it worse. I know vBulletin has been trying to come up with a new editor recently, so hopefully this nesting issue becomes less common.