I would like to know how to configure my VPS so that it's only accessible using
sub.domain.com, and not
domain.com. Anyone know how to do this?
Point the domain.com A record else where and the www. at your servers IP address.
A better option would be to 301 redirect the domain.com to the sub.domain.com so if a user types in domain.com it shows as sub.domain.com
SImple: do not define an IP address for domain.com, only one for sub.domain.com.