Harris scorns religious belief: faith is an "abyss" into which “former atheists” are easily precipitated because they were never authentic atheists, merely alienated religious, eager to return to the faith. The tenor of the vampire image is that yearning for the “emotional consolations” of religion is an irresistible and perverted desire, like the vampire’s hunger for blood. Harris might, without mixing his metaphors any more ludicrously, have written “crackhead intensity” or “the intensity of an alcoholic temporarily on the wagon”, but . . .
"Vampiric intensity" is apparently a modestly popular catchphrase within the narrow confines of fanfiction (mostly about vampires), movie reviews (of romantic actors) and bad poetry (about nothing readily discernible). Googling the phrase with -"Sam Harris" yields 186 results, and eliminating duplicates yields (I think) 40.
Makes you wonder what Sam Harris reads for light entertainment.
All of these are dated 2004 or later, but there is one earlier use, from a 1997 Time review:
Gently, lovingly, at other times with parasitic intention or vampiric intensity, men have turned to women for inspiration. F. Scott Fitzgerald had Zelda, Rodin had Camille Claudel, Picasso had a distaff palette; and Bob Dylan, one of the most intriguing, important, irascible figures in rock, had whom? On Time Out of Mind, his first CD of new, self-penned material in seven years and his most consistently rewarding album since the '70s, Dylan seems to be haunted by an imaginary, unnamed muse who has come and gone, leaving him loveless and listless,...