vaci is the substitute of brū 'to speak' when any ārdhadhātuka affix is to be supplied.,
वक्ता 'he will speak,' वक्तुम् 'in order to speak.' The इ of वचि is for the sake of articulation only. The substitute being like the principal, वच् is Atmanepadi when the fruit of the action accrues to the agent, as ऊचे 'he spoke,' वक्ष्यते 'he will speak.',