Vy˚ is the semi -- vowel (i. e. half -- consonantic) form of vi˚ before following a & ā (vya˚, vyā), very rarely ū & o. The prefix vi˚ is very unstable, and a variety of forms are also attached to vy˚, which, after the manner of all consonant -- combns in Pāḷi, may apart from its regular form vy˚ appear either as contracted to vv˚ (written v˚), like vagga (for vyagga), vaya (for vyaya), vosita (=vyosita), *vvūha (=vyūha, appearing as ˚bhūha), or diaeretic as viy˚ (in poetry) or veyy˚ (popular), e. g. viyañjana, viyārambha, viyāyata; or veyvañjanika, veyyākaraṇa, veyyāyika. It further appears as by˚ (like byaggha, byañjana, byappatha, byamha, byāpanna, byābādha etc.). In a few cases vya˚ represents (a diaeretic) vi˚, as in vyamhita & vyasanna; and vyā˚=vi˚ in vyārosa.

Pali-English Dictionary, TW Rhys Davids, William Stede,