If you are OK with chilean prices, try any restaurant from happycow. If no, there are some tips of staying cheap and vegan:
- Marraqueta is a kind of chilean white bread, that is similar to french baguette. It's vegan. About 70% of consumption of bread in Chile goes to marraqueta. Other kinds of bread contain milk.
- Check supermarket Unimarc. They sell prepared food and it's quite cheap, you can easily find something vegan.
- In any supermarket you will also find canned (or tetrapak) beans and vegetables. With all above you will have an acceptable lunch.