Issue: Change sectionMain to primaryContent

primaryContent does not have a border in default XenForo. VaultWiki used to use a style similar to primaryContent for the blocks you are referring to, but many users complained that it didn't look like it was part of XenForo without the borders provided by sectionMain.

So sectionMain was chosen because it has the closest default CSS to that intended for VaultWiki blocks, and is pretty semantic with how it is used in XenForo:
According to XenForo's description of sectionMain, it is simply "A specially emphasized version of 'Section'." And Section: "Wraps around most content."
Now Section on its own is used quite generously throughout single pages, so the "most" does not necessarily refer to containing a bunch of content in one container, just that most content appears as a descendant of any Section container.
Furthermore this definition does not designate that sectionMain MUST be used to contain all content in one element, but just suggests that the class is used to "emphasize" certain otherwise-Section containers.

Now in general, yes, it is true that most pages only contain one SectionMain element. However, the definition of SectionMain does not explicitly dictate that.

Now there are actually no suitable replacements for how sectionMain is used in VaultWiki. If we were to change it (again), it would be back to a custom class that is a clone of sectionMain, which would be another thing to look out for when styling, and would be another way that VaultWiki DOESN'T integrate with XenForo automatically.

VaultWiki should work fine with the default style of XenForo and many custom styles, if they follow existing naming conventions (like not reversing the meaning of "light" and "dark"), and don't impose restrictions on classes that don't have them by default. However, if you are using a custom style that DOES do this, then it's a simple task to modify the vw.css in that style to work around the conflict, or to use an additional.css -like template to achieve if you don't like modifying the main .css templates (I would recommend the alt template option). Most wiki elements have added class identifiers, like the current example ".vw-block", which can be used as a selector to help resolve conflicts.