I have a question. Are vocals the deciding factor when determining if a band is an extreme genre or not? Such as death metal or black metal.
Can death metal or black be death metal or black without their typical vocal style?
For example. Would, say Cryptopsy's None So Vile with Joey Belladonna's vocal style be death metal?
Or what about Bathory's Under The Sign of the Black Mark with maybe Bruce Dickenson?
More examples I'm sure come to mind.

I'd like to hear what others think about this.

Good question, like there are instrumental bm bands out there, but would they be bm if they had death metal vocals?...well yeah i suppose it would just be blackened death.

It's defnitely a big part of defining a band. I mean, I wouldn't consider Arch Enemy a Melodeath band if Angela Gossow sung like Tarja Turunen or Simone Simmons. Although, their riffage and speed are also key factors in defining them as such.

I consider Metalcore too not only be defined as it is because of the breakdowns and generic riffage, but because of the thin(whiny) vocal style(both distorted and clean).

I'd consider it atleast 50% of what defines a band as they are, if not more. Depending on the situation ofcourse, like the instrumental sound of the band in question is equally important i'd say. I mean, a Black Metal instrumental band still has that atmospheric, dark, tremolo picking sound, I'd still consider them Black Metal.

Vocals are a factor like anything else, but they're definitely not the deciding factor. There are black metal bands with clean vocals that everyone still easily identifies as black metal.

Yeah, indeed. It's one of the factors only.
Sometimes it may not define a band. Blood of Kingu apart from the death metal vocals is considered black metal.
Sometimes it may do. Pretty sure Dimmu is only considered black nowadays due to the vocals

It's one of them but not THE deciding factor in every case, it depends on how strong a certain sub-genre is identified on the instrumental part I guess.
Anyway, I see sub-genres only as a way to guide the listener on what he is about to listen and something we all may or may not agree on, and that's fine. I'm sure there'll be a lot of different opinions coming up regarding your question on switching vocal styles to certain bands.

Sólstafir's album Köld is interesting in this regard. I recall there being a song that doesn't have the characteristic vocal style, but can be considered black metal because of other factors.