Stockholm:

Hermans - Fjallgatan 23 A
Many options on the buffet and a good selection of veg and some vegan desserts. It's casual and cheap by Stockholm standards - about $20 per person.

Chutney - Katarina Bangata 19
Very casual place in the edgier part of SoFo. Also about $20 per.

Primo Ciao Ciao
One in SoFo (generally packed) and another on Kungsholmen. Not a vegetarian restaurant, but lots of good veg pizzas and pastas. About $30 per.

F12 - Fredsgatan 12
Also not a vegetarian restaurant, but they do offer an 8-course (really 10) vegetarian tasting menu that's quite creative. They will also do blind wine pariings, which are quite fun. Expect to spend about $200 per person.