Grail Vessel

The substitute prepared to facilitate the descent of the Holy Grail. In the first three Grail Wars it was merely an object, but in the fourth war the Einzberns sought to gain an advantage in the struggle for possession of the vessel by implanting it with a will of its own and a sense of identification with the Einzbern family.
Although Irisviel’s homunculus body is unmistakably organic, the inorganic Holy Grail was broken down and fused with her organs.
While it’s certainly true that making the vessel required to win the war a member of their own faction is a reprehensible act of cheating, technically their only obligation is to “deliver a vessel to Fuyuki,” which they did. Besides, it’s a war. Is there really that big of a difference between stealing somebody’s possessions and killing them then taking the body?

Fate/Zero material: Fate/Zero Encylopedia

Keeper of the Holy Grail, which is stored inside the womb. The grail in the womb is also known as the Lesser Grail. Irisviel and Illyasviel are homunculus created to be grail vessels. The grail vessel is an outer shell for the Holy Grail that also functions as a mechanism to collect and store the souls of vanquished Servants.

Fate/Zero Animation Guide II: Glossary of the holy grail war II