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 somethimes 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 belive 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.