Error Pages Stop Functioning

Before VBSEO, my awesome error pages were still working,

example of it:

X-Devs | 400 Bad Request

And I had this functioning with htaccess:

ErrorDocument 400 /errors/400.html
ErrorDocument 401 /errors/401.html
ErrorDocument 403 /errors/403.html
ErrorDocument 404 /errors/404.html
ErrorDocument 500 /errors/500.html
ErrorDocument 501 /errors/501.html
ErrorDocument 503 /errors/503.html

Now it doesn't function any more:

http://www.x-devs.com/unexistinglinkhereblahblahignegr

Doesn't go to X-Devs | 404 Not Found

In the vBSEO Control Panel you can choose how to handle 404 errors. Just point it to your same 404.html page using the "Custom" option.

My other error pages don't need this kind of special treatment, and will remain functioning correct?

Yes, that should not affect your other error pages.