Istanbul is an Ottoman port near the Black Sea. Its location in VCO is actually different from the location in real life. In real life Istanbul is actually in the on the north coast of the Black Sea, whereas in VCO, Istanbul is at the small river leading into the Black Sea, at the south coast.
Istanbul is located north east of Athens . Heading north from Athens, you will see the adventure islands Lemnos and Thasos. Once you reach them, there will be a little passage to the north east, go up there, Istanbul is at the end of the passage leading to the Black Sea.