Vampires are really only people that believe they are vampires; some think orthodoxy vampirism is the way to go, and others just occasionally play the role. What a person believes to be true, by virtue of the embraced belief, will become truth for them, whether their reality holds water and life or not. If it does not hold water and life, it will inevitably perish, with or without the person who ascribes to that belief.
The cross, ostensibly for the prevailing number of Christian believers, represents salvation and eternal life -erroneously of course, because the cross is in fact a celebratory representation of the "crucifixion" of Christ, whereas the Atonement of Christ (through repentance) properly represents the celebratory "resurrection" of Christ, and our redemption unto Him; which would you rather adopt as a symbol of friendship to indeed anyone, the cross or the Atonement?
Vampires then, knowing that a house divided upon itself cannot stand, are afraid of the cross because, either way, it represents the immanent demise of their chosen life and consequence following. They know this to be true because of the Grace of God instilling upon their minds, in a still small voice, the veracity of the evil they have chosen to replicate, and they know the fact that good will always conquer evil, with a vengeance that belongs only to the LORD.