There is only one vegetarian restaurant in Cambridge although it does sell a lot of vegan food. It's called the Rainbow Cafe and is opposite King's College. I went there twice ordering the lentil soup with sundried tomoto garlic bread and an elderflower drink on the first day and the chocolate and raspberry vegan cheesecake with pomegranate and cranberry tea the second day.
The cheesecake had a smooth and creamy texture but barely tasted like anything. The biscuit crust was equally disappointing. The tea, however, was very pleasant but as they just added hot water I can't really give them credit for that.
The restaurant itself is in a basement and is quite cramped and can get really hot in the back room as that is where the kitchen is also. There is seating in the front room but apparently they don't use it.
The place has received really good reviews from a lot of different places but I was not impressed. Especially since the prices are around 11 pounds each dish.
I did eat somewhere that was modest and delicious, though. My friend Zach (I think he likes to be called Comrade Zach) invited me and some other friends over for dinner while I was in Cambridge. He cooked an asian stir fry which was delicious. I think this was partly credit to the fact that he accidentally put half a bottle of tamari in when adding the sauces. But it looked great (kind of like an asian risotto!) and tasted great and everyone was going back for seconds.