How do I map a subdomain in wordpress to the primary domain in a multisite setup?

Do you have to have secure.domain.com function or can it redirect?

The tl;dr is that you don’t. The primary domain is the cheese: it stands alone. However…. You can use .htaccess to check ‘If someone is coming from secure.domain.com, send them to domain.com’

RewriteCond %{HTTP_HOST} ^secure\.domain\.com
RewriteRule ^(.*) http://domain.com/$1 [L,R=301]
RewriteCond %{HTTP_HOST} ^www\.domain\.com
RewriteRule ^(.*) http://domain.com/$1 [L,R=301]

That’s about as good as it gets. (Mind you, if your server itself doesn’t redirect www.domain.com to domain.com, it needs a kick in the pants anyway…)