The TeamSpeak Virtual Server Port is used when your server is not “a real server running on its own machine” but rather “a server within a server”. This means that the hardware of the server hosting your TS is not dedicated to your TS server allone, but shared with many other users who have their TS servers running on the same machine.
Because many TeamSpeak servers are running on this one single machine (1 machine = 1 physical adress) there must be a way to distinguish between the different servers. Therefore you do not connect to such a server only with its physical adress (that is: IP adress or hostname), instead you add the port number of the VirtualServer.
That is a connection like:
…just the very same ConnectionString that you use in your real TeamSpeak Client on your desktop machine.