Communion and the Sacred Wafer
As long as we're talking about drinking blood, we should pause to think about the Christian ritual of Holy Communion (a.k.a. the Lord's Supper or Holy Eucharist). Holy Communion is a kind of reenactment of Jesus' last meal with his disciples the night before he was crucified. He ate some bread and had some wine, shared it with his friends, and told them that the bread represented his body and that the wine represented his blood (he knew he was about to die). He also instructed them to remember him whenever they had wine and bread. Christians of almost every sect perform some version of Communion, eating bread or wafers and drinking wine. However, one of the main differences between Catholic Communion and most Protestant Communion is the Roman Catholic belief that, during the rite of Communion, the bread and wine consumed actually change to become the body and blood of Jesus. (This is called "transubstantiation," for the "changing" ["trans"] of substance.)
So when Van Helsing shows up with "Sacred Wafers," what he has are Communion wafers (bread) that have already been blessed by a priest. And since Van Helsing is Roman Catholic, he believes in transubstantiation – that the wafers only look like wafers, but are actually the body of Jesus. That's about as holy as you can get in the Christian tradition.
Why is this important in Dracula, you ask? Well, at its most basic level you could view the Christian rite of Communion as being about gaining strength from consuming someone else's blood. Is vampirism a twisted version of the most sacred of Christian rituals? That makes vampirism pretty darn unholy. And maybe that's why the Sacred Wafer that Van Helsing brings is so effective as a vampire repellant. The Sacred Wafer and vampires are like opposite ends of a magnet – they simply can't touch, according to a fundamental physical (or spiritual) law.