I've always seen it as the faith, not the symbol itself. 
There have been marvel cases where specific vampires who were/are religious in their mortal lives are more affected by the symbols of their faith. (it came up in the story arc when storm and gambit were vampires and converting all of the x-men, standard crosses didn't work on gambit but voodoo made the Cajun flee.) [u][i][b]IF[/b][/i][/u] I remember right, there was an incident where a vampire was warded off by an empty gun because the officer had faith that there were rounds in the revolver, he didn't know they had all been shot.