Dear Chris Undery (L&OD),
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, it seems that you are trying to add a vertical section to a couple of SharePoint pages, however, the option is greyed out in existing pages and working in newly created pages.
If the understanding above is right, as per the search and test on our side, generally, Vertical sections are currently available only on the right side of the page, and we cannot have a full-width column and a vertical section on the same page.
For your reference: Add sections and columns on a SharePoint modern page - Microsoft Support
We've also done a quick test on our side, we can add vertical section on right side of the page without full-width section as expected. If we try to add section to page with full-width section such as adding in communication sites homepages, Vertical section would be greyed out. Below is the screenshots of our test for your reference.
Given this situation, we'd recommend you go through the pages to make sure the full-width section is not present when trying to add a vertical section.
We've also found a similar thread for your reference: Modern: missing vertical column option in some sites - Microsoft Community
If the issue persists, we totally understand the frustration and apologize all the trouble caused by this problem. Since issue happened on different pages, more resources need to be involved in for further investigation.
Given this situation, we'd recommend you contact your Microsoft 365 admin if you are not the admin, and create a support ticket from Microsoft 365 Admin Center>Support>New Service Request as guided in this article Get support | Microsoft Docs.
The support team has higher permission to remotely help you investigate the issue and also collect logs when issue happens. They have more resource and the correct escalation channel for this case. This is also the more efficient way in handling this kind of issue for you.
For your reference: How do I find my Microsoft 365 admin.
Thanks for your time and cooperation! If our understanding of the scenario is not consistent with yours, feel free to point it out.
Sincerely,
Rhoda | Microsoft Community Moderator