The vegan bokit at Oval Farmer's Market is pretty much my favourite food on the planet, street food or not http://www.bokitla.com
It's a fried bread and they fill it with aubergine and avocado and the most amazing sauce I've ever tasted.
Oval farmer's market is opposite Oval tube on Sat 10-3. Prob not one you'd make a trip to if you're only here for a few days as nothing of interest nearby - though not too far from the vegan mecca of Brixton. Plenty of street food there on a Saturday and you might be here for one of the Brixton Vegan Meetup walkabouts - have a look on the London Vegan Meetup page. There is sometimes one in Brick Lane too. The meet up info gives details of the vegan street food available at both places.