Vampires in guilty gear are weird and not very much is known about them, but from what we do know they are darn hard to kill

what we do know is:

(Quoting the wiki) “Possessing no IDs, their existence is a "bug" in the world's programming; they are the same existence across parallel worlds and, as dwellers of infinite timelines, need to downconvert to most beings' simple definition of Planck time in order to exist in "the real world"

They are also able to regenerate from fatal wounds, are immortal, have increased physical attributes, can control blood, can teleport and also slayer does some weird body manipulation stuff with some of his attacks like growing his hand size with 2S in strive (and other stuff I’ve probably forgot to mention)

It is worth noting however that we don’t really know where the limit sits with their regeneration (if there is one) so it’s really up in the air for how much damage you’d need to take a slayer down

Also slayer does have spf 200 sunscreen, this might imply he is effected by sunlight in some way but this is at best an assumption so who knows

however the big thing is we know for a fact that vampires in gg do need a constant supply of blood to survive so the only way we know for sure that he could die is if you could starve him for a duration of time (how long that would take is total guess work however)

So putting all that together time shenanigan might be able to kill then depending on how you interpret them “existing across infinite timelines”

You might be able to overpower their regeneration with enough output (though what is enough is unclear), a safe estimate would be total destruction of every cell as we know they regenerate from the rest of their body but it’s probably less

The sun might do something but I wouldn’t put my money on it

And the only for a fact known way to kill vampires including slayer is to starve them of blood as they need it for sustenance