Ok uh, not much of a difference from the title:
Would we have to update our mods when Vassals comes out? (hey it's underlined this time!)

I think every mod maker, me included, hates updating his files! Now would vassals accept the 1.61 mods or we'd have to figure out the differences again then implement them into our mods?

Yes, mod makers will have to update their mods to be compatible with the Warlords expansion. This includes having to update the SDK files and recompiling DLLs.

Plus you have to consider game balance. Vassalage might change the game balance of Civilization 4. (Or it might not!) Or vassalage might have unintended effects when combined with something you added to the game.

So even once you have solved compatibilities issues... it's also a good idea to examine game balance.

Bad news for me :(