I just went here today for lunch. I found it by accident when I was walking around SOHO. They had a huge sign that said "Hawa Whispering Vegetarian." This restaurant was not on the 3rd floor- it was on the basement level. From the main road, you take a flight of stairs down, or you can find if from inside the building by going downstairs.
It is very very hidden- the lights were not on, the windows were covered with curtains, and I did not see a single person inside, but I went in to give it a try. They were open, and had food all ready. They had a simple buffet all laid out, and it only cost 20 RMB for all you can eat. It was a big restaurant with a few people inside, very quiet. It did not seem like they were making an effort to attract people. The staff was very friendly, and spoke English. The food was very good, and tasted home-cooked.
I don't understand how they get any business with the exterior looking very closed up, but I recommend you give it a try! This is the CHEAPEST vegan food I have found.
This is their address according to their business card:
B-122 West tower SOHO Shangdu, No.8, East Bridge Road, Chaoyang District, Beijing.
Cons: looks uninviting from the outside