Why am I dealing less damage due to fatigue?
Geralt deals less damage the less vigor he has availible. This happened in the base version of the game too, but as of Enhanced Edition, it is explicitly mentioned in the combat log.
If you have zero vigor, your attacks deal 50% damage. If you have full vigor, your attacks do 100% damage. If you have somewhere between those, each point of vigor is worth 50/N % damage, where N is the total number of vigor points you have.