As mentioned before, most Bengalis aren't strict vegetarians. Pretty much all of the pure Veg places are run by South Indians (who mainly are strict vegetarians).
There are other places mentioned in the popular travel guides, but these places are purported to be pure-veg and are at least somewhat close to the hotels (above).
Junior Brothers 18B Park Street (upstairs) serves inexpensive thalis.
Vineet is a high class place, located at the famous Air Conditioned market (AC market) near the intersection of Chowringhee Road and Shakespeare Sarani
Jyoti Vihar is a snack type of place, near the US Embassy on Ho Chi Minh Sarani
Friends Home is located near the New Empire cinema and Cyber Empire internet cafe on Newmarket street in the Sudder Street area.
There are some other pure-veg places mentioned in the travel guides found on the
Travel Planning page.