I've been in two cities that I remember vividly for street vendors, and those are Mexico City and Venice Beach. These are two completely different cities and have different things for sale.
Venice Beach, CA has a large strip that is basically a flea market, people selling art, doing portraits, selling goods etc. Right across the walkway are 'legitimate' businesses selling food and touristy stuff. I think they compliment each other - the flea market side is a draw for people, and the other side has lunch when you get hungry (and also a botox clinic - hmm). This seems to me like a highly successful integration of a relatively unmonitored market-type space and your average pedestrian district. I LOVE Venice for these and other reasons.
Mexico City is amazing because everywhere you turn, at least in public space, there's someone selling something (usually food). If you want to go to a restaurant and pay regular prices, okay, but if you eat street food you save a lot of money and it is often delicious. I feel this compliments the 'legitimate' restaurants because if you feel like eating fancy no one's stopping you, but the poor and those with hardened intestinal tracts have their options as well. I also love Mexico City, and this is one of the reasons.
I find places with street vendors to be more alive and weird in that good, urban way than sterile streets without them. Also, using Jacobsian logic, they put more eyes on the street and thus could help to make these areas more safe (provided the stalls are not run by thieves and pedophiles, etc.). I couldn't support them more.