If you want an honest to goodness vegetarian/vegan food without the fancy trimmings you will find in the expensive vegetarian restos in Cusco, then head to Govinda Lila's vegetarian restaurant. It is a corner stall in the Mercado San Blas. For 6 soles you will get a hot drink, a big bowl of soup and a plate full of rice with vegetarian side dish. Nothing beats it.
Cons: located in a market so don't expect fancy decorati