There is vocalisation of the half-vowel of the reduplicate of vyath in the liṭ (Perfect Tense).,
As विव्यथे, विव्यथाते, विव्यथिरे ॥ This ordains the vocalisation of य, which otherwise would have been elided as being a non-initial consonant 7.4.60. The vocalisation of व् is prevented by 6.1.37. Why do we say in the Perfect? Observe वाव्यथ्यते ॥,