При использовании протокола HTTPS заголовки хоста проигнорированы (по крайней мере, на IIS 6). Если Вы не захотите, чтобы это явление произошло, то Вы должны будете присвоить www.site1.com специализированному IP-адресу, не используя "Все Неназначенные", и не используете другой сайт SSL на том же IP-адресе.
You cannot set the reverse DNS yourself -- you have to ask your VPS provider to do that.
With an IP of 159.x.y.z
the reverse DNS is basically a lookup of z.y.x.159.in-addr.arpa.
, as described in the Wikipedia article.
With a local BIND you might get a reverse DNS configuration on your own server, but nobody else would ask your server and see it. So it has to be configured by the network operator who manages the IP address range and has the DNS zone delegation for it (most probably the VPS provider or its upstream ISP).