You can say "there are some vendors in the street" and it sounds normal. But if you were to say "they are some vendors" it sounds weird. It's pretty firmly more natural to say "they are vendors."
In the case you mention, I note that Huddleston & Pullum in their Cambridge Grammar of the English Language specifically call ‘some’ an existential determiner. As is usual with them, this is an insightful observation: you use ‘some <noun>’ specifically in order to say that there exists some set of things drawn from some larger set or in some configuration. You say ‘there are some vendors in the street’ because you’re drawing attention to their existence in the street (and indeed ‘there is/are’ is the prototypical English expression for existentials). You don’t say ‘they are some vendors’, because the use of ‘they’ implies some specific group of vendors already known to both speaker and listener — highlighting their existence again feels weird.
(Of course, this analysis is still incomplete. At some point you simply have to list all the specific ways in which ‘some’ is used — which, indeed, is exactly what Huddleston & Pullum do. Furthermore I’m certain that there is some discourse context in which ‘they are some vendors’ would be natural, even though I haven’t managed to think of one just yet.)
"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.)