After you activate the artifact , you still need to sacrifice 3 creatures.
The artefact completes the "channeling" part of a ritual, which for most rituals is the full preparation process of the ritual, in the case of the Summon Vampire ritual, it only completes the first 50% which is usually channeled by cultists so you will still require the 3 sacrifices are normal.
Put simply, with this particular ritual (since it differs from the others), the artefact will get you from 0-50% instantly ready for the sacrifices to be made.
As Nutter has quite rightly identified this is intended behaviour and not a bug, the Incantation ritual will complete the channeling part of any ritual but not any additional requirements (such as the sacrifice here) I would agree that this is not appropriately telegraphed however and I feel this is a problem with the Vampire ritual more than anything. I'll put it in review so we can discuss design in 2018 but there's only a remote chance changes will be made to this, it all depends if we can afford the resources to do something about it.