The affix khac comes after the verb yas 'to curb' when the word vāk 'speech' is in composition with it as an object and the sense indicated is that of a vow.,
The word व्रत means a vow taken according to the rules of Dharma sastra. Thus वाचंयम आस्ते 'he has taken the vow of rigid silence'.
Why do we say 'vow'? Otherwise the form is वाग्यामः 'restraining speech' 6.3.69.,