Vakula Devi is the foster mother of Lord Venkateswara. As per the legend of Tirumala, it dates back to Dvapara Yuga when Yasoda, the foster mother of Lord Krishna (avatar of Lord Vishnu) complains to him that she couldn’t witness any of his marriages. To this, Lord Krishna replies he would ensure she would get such opportunity later in Kali Yuga.
In Kali Yuga, Lord Vishnu adorns the world as Lord Venkateswara and Yasoda is reborn as Vakula Devi, foster mother of Lord Venkateswara, to arrange his wedding with Padmavati the daughter of King Akasa Raja. Thus Vakula Devi fulfills her wish to witness the kalyanam (wedding) of Lord Venkateswara.