Silverstripe 3.4 - Frontend works fine, but leading slash removed from /admin and /admin/pages - apache

Question

I've inherited an existing Silverstripe 3.4 site and am setting up a new Vagrant VM for it. It has an /app directory that Grunt copies into /public-www on build. Composer pulls everything else into public-www.
Website loads fine. Home page, and subpages multiple levels deep like http://mysite.local/dev/tasks. However it's not possible to load the admin login page.
http://mysite.local/admin redirects to http://mysite.localadmin/pages
(slash removed)
http://mysite.local/admin/pages redirects to
http://mysite.localsecurity/login?BackURL=%2Fadmin%2Fpages (slash
removed again after redirecting to security this time)
http://mysite.local/security/login?BackURL=%2Fadmin%2Fpages redirects
to a 404 (Silverstripe type, not apache)
http://mysite.local/Security/login?BackURL=%2Fadmin%2Fpages (note manually changing to uppercase "Security") loads the login form, but the default dev login defined in _config.php does not work.
Surely an Apache issue. The .htaccess file is the same that is running live right now without issue. But I can't see the problem in either. The following is my apache config (/etc/apache2/sites-enabled/default.conf)
ServerName local
DocumentRoot /var/www/html
<IfModule mod_fastcgi.c>
AddHandler php56-fcgi-www .php
Action php56-fcgi-www /php56-fcgi-www
Alias /php56-fcgi-www /usr/lib/cgi-bin/php56-fcgi-www
FastCgiExternalServer /usr/lib/cgi-bin/php56-fcgi-www -socket /run/php/php5.6-fpm.sock -idle-timeout 1800 -pass-header Authorization
<Directory "/usr/lib/cgi-bin">
Require all granted
</Directory>
</IfModule>
<IfModule mod_fastcgi.c>
AddHandler php71-fcgi-www .php
Action php71-fcgi-www /php71-fcgi-www
Alias /php71-fcgi-www /usr/lib/cgi-bin/php71-fcgi-www
FastCgiExternalServer /usr/lib/cgi-bin/php71-fcgi-www -socket /run/php/php7.1-fpm.sock -idle-timeout 1800 -pass-header Authorization
<Directory "/usr/lib/cgi-bin">
Require all granted
</Directory>
</IfModule>
<IfModule mod_fastcgi.c>
<FilesMatch ".+\.ph(p[345]?|t|tml)$">
SetHandler php56-fcgi-www
</FilesMatch>
</IfModule>
<Directory "/var/www/">
AllowOverride All
</Directory>
<VirtualHost *:80>
DocumentRoot /var/www/html/mysite/public-www
ServerName mysite.local
<IfModule mod_fastcgi.c>
<FilesMatch ".+\.ph(p[345]?|t|tml)$">
SetHandler php56-fcgi-www
</FilesMatch>
</IfModule>
</VirtualHost>
<VirtualHost *:80>
DocumentRoot /var/www/phpmyadmin
ServerName phpmyadmin.mysite.local
<IfModule mod_fastcgi.c>
<FilesMatch ".+\.ph(p[345]?|t|tml)$">
SetHandler php56-fcgi-www
</FilesMatch>
</IfModule>
</VirtualHost>

Answer 1

I found the cause of this weird URL redirection. The /mysite/_config.php file had this in it:
Director::setBaseURL('http://mysite.local');
It looks like it was there to get around errors when /dev/build was run from the command line, which I have since seen show up, but at least now admin works. I tried adding a trailing slash, and that certainly did fix the URLs, but then I couldn't log in for some reason. Remove it entirely and I can log in.
Note those errors are complaining about this:
You probably want to define an entry in $_FILE_TO_URL_MAPPING that
covers "/var/www/html/website/public-www"
Despite exactly that being right above the setBaseURL() line I removed.
global $_FILE_TO_URL_MAPPING;
$_FILE_TO_URL_MAPPING['/var/www/html/website/public-www'] = 'http://mysite.local';
Which I think is in the wrong place in /mysite/_config.php. After moving to _ss_environment.php, all is well there too.