vassalstate

Hi everyone! Just a quick question. When do you unlock the vassalage option in the "trade" screen when talking with leaders? Do you just need to enter the Medieval era? Or do you need any other requirements? Asking because i got to the medieval era, but the option is still greyed out. So not...

I posted this up in the 2K games forums some time ago, but really want it to get attention, as this is very important to me (Link on 2K games forums: https://forums.2k.com/showthread.php?4281037-Please-consider-Vassalage)... Civilizations IV had a cool feature that I still sorely miss, where...