Vaishnavi is wearing Vanamala (flower garland) and is sitting in Padmasana. She is framed by a prabhavali (partly broken).
Vaishnavi is one of the Saptamatrikas (seven mothers) and she is also the Sakthi of Vishnu.
This Vigraha possibly dates back to the Pala period (10-12th century of CE) and is from Bengal/Bangladesh area. The dealer mentioned it is from the Mainamati area of Bangladesh. The vigraha is about 6 cm in height.
The peaked crown, long Gada and the patterns on the Chakra go with the regional attribution. As is common with Pala bronzes the back is not worked on.
There is a similar bronze in Bengiya Sahitya Parishad museum (PM-BSP-WB-33651).