This is an intersting question you pose:
Chicago has next to zero street vendors.
I'm sure there is a eral answer out there somwhere, but I always suspected a food-safety/civic beautification bias. There just aren't street vendors like in NYC. A handful of Latino icecream/popsicle vendors circle the city endlessly throughout the summer months, but that's just about it.
This came up on another post ("what would you like to see in Chicago?") I know there are some strict rules (I don't think it's impossible). But I always thought it was due to Ald. Burt Natarus imposing his will on downtown. He was voted out in the last election, though. I wonder if the City Council would be willing to change?
"Next to zero"? "A handful"?
We're talking many hundreds. Maybe you just don't make it out to the rest of Chicago, but in Pilsen, Humboldt Park, Logan Square, Rogers Park, Little Village, Albany Park, etc, tamaleros, eloteros, palateros, and more constantly roaming around in the summer.
Then there are the african and caribbean lunch trucks that are a fixture of the loop and streeterville areas; the maroon van near Palmer Square Park; the "La Esquina de Sabor" truck and its kin parked all summer in Humboldt Park; and all of the food at the Maxwell Street Market.
To a visitor from out of town who is lucky enough to be here on a Sunday, I strongly suggest searching here and at lthforum.com for more information about the Maxwell Street Market, and heading down there and sampling some street food that would put New York City's to shame.