Vampires! They're not just evil bloodsuckers lusting after your blood. They're symbols. Allegories. Bleak avatars of sense hiding behind the blank windows of silence. They mean stuff: listen or lose your soul.
So, we know vampires are meanies. But what do those meanies mean?
Well, most obviously, the vampires stand in for evil, corruption, nastiness, filth. They're symbols of the town's own horrible secrets. When Jimmy and Mark pull Roy McDougall out of the space beneath his trailer, and he starts to writhe and smoke and jerk, the whole thing is "almost unbearable" to watch (14.927-931).
The unbearableness is uncanny and unnatural, but that unnaturalness can be seen as a human unnaturalness: McDougall's marriage, the novel shows earlier, is an ugly thing, mostly loveless and helpless. His wife even beats their infant child.
So McDougall's sick jerking is in some sense because he's a vampire in the light, but it's also a symbol of his marriage itself, in which he (and his wife and his child) thrash about senselessly. When Jimmy first sees the three pairs of feet lined up in the hole, he thinks that they look like a sweet "Family scene," a horrible appearance of normality hiding unholy evil (14.918).
But that's the case not just for the vampire McDougalls; it's also the case for the pre-vampire McDougalls. They always were vampires; actually turning into literal vampires just makes that clear.
Similarly, the bitter, ugly, vicious Ann Norton vampire is just a reflection of Ann's pre-vampiric bitterness and meanness. Dud Rodgers was always a creepy, sadistic guy who lusted after high-school girls; being a vampire doesn't change that. Lawrence Crockett was always a secretive, grasping bloodsucker. The vampires are the townspeople, and the townspeople are the vampires; nothing changes in Jerusalem's Lot. "That's why he came here," as Parkins Gilman says (14.1088).
The thing is, though—Parkins isn't the good guy. Nope: the good guys—that's Ben Mears, the noble author, and Mark Petrie, the even nobler spunky kid—are horrified by what he says. And lots of people in the town aren't evil—Susan Norton, Mark himself, Matt Burke, Susan's dad, Matt's dad. Lots of folks are small-town nice, or just human. They don't come across as remorselessly evil or horrifying.
Parkins is wrong; they don't all deserve to be turned into vampires.
But then, what's the metaphor? If the book isn't about small towns being evil, what's it about? Or isn't it about anything except how much fun it is to watch a not-entirely-innocent small town spiral into hell—to see the good, hardworking folk (the town gossip, the deceptively slow-talking lawman, the Irish priest) first exposed as maybe not so good, and then dragged into unholy darkness?
Maybe it's not just the townspeople who "pro'bly like bein' vampires," but the readers, who get to feed gleefully on corruption and degradation and shudder delightfully as the boring little town turns into a pit of filth. And then the heroes get to go around and righteously murder all those boring little townspeople—for their own good, of course.
"They spill each other's blood with great vigor," Barlow, the evil vampire says (10.324). He's referring to Americans, and small-town Americans especially, but he could also be referring to the author and the folks turning the author's pages. Small towns and vampires go together not so much because small towns are evil, but because spilling blood with great vigor is a blast. It's not a metaphor but a spectacle. Crack some popcorn against your white, sharp incisors, and enjoy.