"Redirect Loop" when creating a multisite
Assuming you have ServerName and ServerAlias properly defined in Apache, I suspect you have the default canonical URL redirects enabled in your .htaccess.  Make sure this stuff is all commented out:

  # If your site can be accessed both with and without the 'www.' prefix, you
  # can use one of the following settings to redirect users to your preferred
  # URL, either WITH or WITHOUT the 'www.' prefix. Choose ONLY one option:
  #
  # To redirect all users to access the site WITH the 'www.' prefix,
  # (http://example.com/... will be redirected to http://www.example.com/...)
  # uncomment the following:
  # RewriteCond %{HTTP_HOST} !^www\. [NC]
  # RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
  #
  # To redirect all users to access the site WITHOUT the 'www.' prefix,
  # (http://www.example.com/... will be redirected to http://example.com/...)
  # uncomment the following:
  # RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
  # RewriteRule ^ http://%1%{REQUEST_URI} [L,R=301]


Also make sure you don't have any other Apache rules that are doing something similar, and that you don't have any modules (eg, Domain Access or Mobile Tools) or custom code that would do a redirect.