Hi jaimebien,
Hope it's not too late for your research assignment. His name is Bill - he was featured in the Vendor Profile in an issue a few months back. If you can track that down you'd be able to get his story.
I've talked to him on a number of occasions - he's as nice as pie. If you have a little chat to him I'm sure he'll open up. From what he's told me, he sells around USyd as result of exclusive permission from security - no other Big Issue vendor can sell at USyd. One of his conditions is that he can only sell at entry/exit points of the uni - no haggling of customers (not that Big Issue vendors would do that) and that if he's asked to leave by uni security he must do so.
So for BlueCatBulletCD saying, "He makes me feel guilty for not buying it, twice a day, 5 times a week" and bio_nut's "I wish he'd fuck off" - well, that's your problem - deal with it, you fucktards. Bill is nothing but polite.
As for approaching him, don't offer lunch or sandwiches or whatever you suggested - just go up to him, buy an issue and stay around for a chinwag. Big Issue vendors are used to students/researchers asking them questions. If you want a more in depth look into it, though, I'd suggest you speak to the NSW Office.
Hope that helps.