Originally Posted by
Steward Denethor II
Here is another thing. As the Byzantines (or Romans) on turn 1, you can besiege the Serbians with 2 regiments from Nis. Then send your diplomat immediately to the Serbian general hiding in the woods. Offer them a gift(s) of 100 gold, or offer them map information. To the map information they will respond with Appreciative and say "Give us map information/ Accept or we will attack". You should decline this and your diplomat does not suffer. You can then offer them map information again, repeating this process.
After some diplomatic activity, their priorities will change from Unknown to Peace. This can be 1 or 2 activities, but is sometimes a little more. Then you can demand they become a vassal, and you should probably offer map information so they are more likely to accept. They are happy to accept this offer, and you have a fellow Orthodox vassal without a single battle on your very first turn! I bet they could be a great buffer against the Catholics while you fight the Muslims.
It does not seem to work with Castile and Portugal, however, since there is much less of a power imbalance there.