This is my first time seeing sewerslvt referred to as "daycore."
Like Bobby said, daycore seems to be the opposite of nightcore, but that would imply doing nothing more to the source material than simply slowing it down/pitch shifting down a few octaves.
As you can tell, sewerslvt's work is heavily based in Drum & Bass, and self-describes it as such in their bandcamp page, along with "breakcore," a DnB-adjacent subgenre.
https://sewerslvt.com/album/sewer-slvt-ep
As such, I wouldn't even necessarily call "daycore" a real genre, considering it's almost exclusively a late reaction to nightcore and production seems sparse at best. It doesn't even have a Wikipedia page (circumstantial evidence, I know).
But as far as it's relationship to vaporwave goes, the connections are coincedental. since nightcore is fast, daycore must be slow, which just so happens to be a common theme in classic vaporwave tracks. There's also the anime connection, which is also mere coincidence due to nightcore's focus on it.