Are virtual servers and port forwarding the same thing?
Some routers - eg: D-Link's - use the term 'virtual server' for port forwarding where you can divert specific WAN ports to local machines with translation of port numbers.
Example: WAN (public IP) Port 9422 -> Local machine 192.168.0.1 port 22
In this context, the term has nothing to do with machine virtualisation and 'virtual server' and 'port forwarding' do often mean the same thing.
If there is a differentiation between the two terms, it's usually that 'port forwarding' means that the public port and the local machine port to which it is directed have to use the same port number.
Example: WAN (public IP) Port 22 -> Local machine 192.168.0.1 port 22
Some routers provide a means to appoint one computer on your LAN to receive all incoming connections initiated from the outside. This is referred to using a number of terms and phrases such as "True DMZ†" or just "DMZ - although I don't recall "Virtual server" being one of them.
The term "virtual server" usually means something completely unrelated - the means of hosting one or more guest operating systems concurrently on the same single computer - usually with a host operating system but sometimes with a bare metal hypervisor.
Port forwarding allows you to direct incoming traffic to many different servers on your LAN, depending on the connection's destination port number. For example port 30122 might be forwarded to port 22 on server A, port 30222 might be forwarded to port 22 on server B.
It seems to me the manual for the D-Link DIR-655 uses the term "virtual server" for port-forwarding where the internal and external port numbers are the same. It uses "port forwarding" for port-forwarding where the external and internal port-numbers differ. So far as I know, this use of the term "virtual server" for port-forwarding isn't used generally and is likely to cause confusion with virtual servers in the context of virtualisation.
† Personally I believe that to qualify as a DMZ, the servers in the DMZ should be prevented from making connections to computers in the LAN outside the DMZ. Most low-end routers don't provide this level of protection.