How do I force the FQDN, http://server.domain.com, with a standard Vibe install?

We have users that use http://server and others that use http://server.domain.com.

This causes a user to have to re-authenticate when a link is sent using the other method that they are currently logged in as.

example: I am logged into vibe using http://server.domain.com and get sent a link from another use that is logged in using http://server. When I click the link I have to login again; and vice-versa if I were send a link back.