I’ve written about this a lot – not just on this Board, but elsewhere, too, including one book and my own website (link below).
In brief – there is no traditional belief in sunlight hurting vampires. The closest folklore comes i this is in asserting that vampires are Creatures of the Night, and don’t come out in the daytime. But even Bram Stoker, who codified our “facts” about vampires, didn’t have sunlight hurting them. Harker uncovers Dracula’s coffin during the day without hurting him, and later Dracula walks around London in broad daylight.
The first instance of a vampire being dissolved in daylight is Fritz Murnau’s unauthorized adaptation of Dracula, Nosferatu. But Murnau made up his own rules – as most people do when making up creative works about vampires. His Count Orlock dissolves in daylight, but it’s probably because it’s a nifty special effect, and it falls in neatly with his identification of the Count with sickness and the plague, and sunlight is a cure for infections. (In the film, Nosferatu is blatantly shown as being reflected in a mirror – so Murnau was ready to make more than one change). A lot of places leave it at that, saying that the trope had been invented, so there it was. But Stoker’s widow, who resented the film (both because it hadn’t gotten permission and because she wasn’t getting any royalties, which she desperately needed) took the company to court, and tried to destroy as many copies of the film as she could. she didn’t get them all, but she did manage to keep it out of circulation for a long time. So it couldn’t have influenced pop culture ideas.
In Cark Dreyer’s Vampyr, for instance, the titular vampire is staked (with an iron stake) in what appears to be broad daylight (although the film plays with reality a lot. It might be night time depicted as broad daylight). But it’s the stake that kills her. H.P. Lovecraft’s The Shunned House has the vampire destroyed by being dissolved in acid. In all American movies the vampires were staked.
It wasn’t until WWII that vampires started dissolving in sunlight – Universal’s Son of Dracula, House of Frankenstein, and House of Dracula showed this. Curt Siodmak, who was born in Germany and worked in German film before emigrating to the US, worked on these and wrote scenarios, and I suspect that he remembered Nosferatu and the sunlight, and introduced “death by sunlight” as a non-bloody, “clean” death that could be shown on camera (the stakings were all off-camera in the Universal films), something important during wartime, when it was better not to remind theater patrons of the real bloody deaths taking place in WWII. the 1943 Columbia movie Return of the Vampire (which was effectively a sequel to Dracula, even starring Bela Lugosi, although they had to give his character a different name) also ended with the vampire dissolving in sunlight.
Then in 1958 Hammer films made Horror of Dracula and revived the trope of Death by Sunlight. With three different movie studios showing it, the idea became canon, and now it’s as inextricable as anything else in pop culture vampire fakelore.
The notion that vampires were vulnerable to ultraviolet light started growing up after 1970, finally showing up in films and comic books and literature, so that by 2000 it was itself an established trope. The rationale seems to be that – hey, vampires can walk around under artificial light in the evening all they want and they don’t dissolve. What’s different? The higher levels of ultraviolet light in daylight (and the need for sunscreen to avoid melanoma) seemed a plausible explanation, with more than one vampire using sunscreen to protect itself.
So here we are, with the current vampire Fakelore being that ultraviolet light is kryptonite to vampires. If we accept that as a starting point, then, yeah, UV laser light ought to be deadly to vampires, like the UV LEDs used by characters in Christopher Moore’s off-the-wall vampire trilogy. But visible lasers probably won’t affect them. (On the other hand, fire does, so who’s to say that Q-switched mode-locked laser putting out huge pulses of frequency-doubled Nd:YAG light wouldn’t plough a hole through vampire skin?). But low-output UV lasers might do so with less trouble. So use a frequency-quadrupled Nd:YAG laser or a HeNe laser or a Gallium Nitride diode laser or an Excimer laser and go vampire hunting. The diode lasers are probably your best bet – you can build them into “laser pointer”-style hand-held battery-powered devices that don’t need cooling water or heavy electrical cables.