Since Green Deli Cafe is located in the very centre of Sofia and has pretty decent opening times, I was there three times to eat.
As a soup lover I liked the range of fresh soups (five or six), which can be eaten on the spot or as take-away. At least some are always vegetarian and/or vegan; sometimes they ask you if you want to have some cheese or bacon in it which is available separately. Note that soups are only on display on weekdays not on weekends.
Apart from some salads and cakes (among them freshly baked muffins) there is a big fridge with sandwiches, more salads and desserts. Vegan options are clearly labeled as vegan. For coffees there is soy milk available.
Despite having tables on ground floor and upstairs as well it can easily become crowded, even if many costumers only buy things for take-away.
If you are hungry or have to bridge some time Green Deli Cafe is a decent choice.
Cons: crowded, still lots of non-veg. stuff