There is not vocalisation of the semi-vowel of vaś, when the affix yaṅ follows.,
The word यङि is understood here. As वावश्यते, वावश्येते, वावश्यन्ते ॥ Why do we say \when यङ् follows\? Observe उष्टः, उशन्ति ॥ See (VI. I. 16) to which this rule is an exception.,