The problem is the real domains are running on a different production server.
So I want to set up temporary domains or subdomains on the new box test it out and then tell my ISP to move the domains over to the new box as soon as I'm ready. This is to have little or no downtime.
So what I've done now is go ahead and use the real domains in the sites tab.
This ofcourse wont work because the domains resolve to my other server IP.
So like I said I got it work by maually adding the ServerName in .vhost like this
real-domain.newhost.com which works great.
But ispconfig3 likes to overwrite my manual changes everytime i edit the site in sites tab.
Maybe this helps anyone testing out a new server before moving.