All Ubuntu has is
Include sites-enabled/ at the end of apache2.conf, which means 'concatenate everything inside sites-enabled then parse'. Not far different from the default configs on my other servers really. The
apache documentation appears to agree, there's no extra magic to it.
I'm beginning to think I just fat-fingered something in my first attempts and got myself confused, stuck in a blind alley. A combined config should still work hypothetically. But I've been fighting with this so long I'm just thrilled that it works at all.
I will further refine these vhosts now that I have something that works, thank you for your suggestions. I kept them intentionally as simple as possible to rule out other problems.