Vassal is a UC term, and is not an implemented system on GC. On GC, you can attack somebody, and if you are both terran/a. miner you can choose to do an Enslave Attack. If you destroy 60% or more of their fleet, and they lose more pr total than you, then they are now your slave empire. They send you automatically 15% of their cash income to your tribute reserve. You can choose to send reinforcements to your vassals, and ships will be sent from your stack to them when they are attacked sometimes under certain conditions, see the Manual and FAQ for a ltitle more info.