Like any PlayMesh hospital, you cannot use the Vampire II Coffin if you are at full health. There will just be a line of text telling you that you "don't need the blood sacrifice." If you are at a lower health level then there will be a button in the center and a cost above that.
The blood cost for using the Vampire II coffin will depend on how low you are and what level you are at. Toward the top of the Vampire II Coffin view will be your Bank Balance. This is just a listing of how much blood, or money depending on how literal you want to be, you have in the Vampire II Blood Bank.
You have to use blood from the Vampire II Blood Bank in the Vampire II Coffin to pay the bill. You cannot just use spare blood. Make sure that the cost of using the Vampire II Coffin is not more than what you have in your Vampire II Blood Bank account.