A Virtual Server is a standalone virtual image that contain the operating system (Windows, Linux etc). No firewall or any protection on infrastructure layer. You can only rely on OS firewall. Connection type is also direct connection.
So what is different between Virtual Server and Virtual Data Center?
In term of support, resources and price, there are no different between two of them. Virtual Data Center is more likely the management environment that acts as a container for many Virtual Servers. As management, it has more control to access features compare to single Virtual Server only.
In term of network and security, No firewall or any protection on infrastructure layer. You can only rely on OS firewall. Connection type is also direct connection. Compare to Virtual Data Center, you can specify what connection, ports that allowed to access your server because VDC has feature for security on infrastructure layer.
Please refers to this table comparison to find more about VDC vs Virtual Server.