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.)