"They are some vendors" seems natural to me, suggesting that the group of vendors is known to the person answering.
"The street vendors have decided to oppose the ordinance."
"But I thought Mike and Jacob said they support the ordinance. And Julia! Aren't they vendors?"
"They are some vendors, but most vendors take the opposite position."
"They are some vendors, not all vendors" is natural.
"They are vendors, not all vendors"is equally natural.
Written without stress indicators, most dialog can sound unnatural.
No it isn't. For example, I am a native speaker, and I have no idea what you wanted that sentence to say. There is no available contrast between the emphasized words, leaving me mystified as to why they're both emphasized.
Looking up through the context, I think what you want is "They are vendors, but they aren't all vendors." That's still not a felicitous way to express it; you'd be better off with "They are vendors, but they aren't every vendor." (Because "They aren't all vendors" is most naturally interpreted to mean "not all of them are vendors", with all as an adverb, as opposed to "not all vendors are one of them", with all as a determiner.)