Being suspicious because vazco_topbar wasn't working right (and since no users are accessing the site right now anyway), I thought I'd try turning vazco_topbar and vazco_topbar_template off.
And now my site only displays a grey screen. D: Ran the upgrade.php script but no change.
Any way to turn a plugin on with a quick edit of the database?
For future reference to anyone who comes along this way later: the issue was one of permissions, specifically permissions for the datastore directory. :)