vī is the substitute of the verb aj 'to drive', 'to lead', when an ārdhadhātuka affix follows, with the exception of the affixes ghañ -- bhāve [[3.3.18]] and ap -- samudorajaḥ paśuṣu [[3.3.69]].,
प्रवायकः 'driver,' प्रवायणीयः.
Why do we say 'with the exception of घञ् and अप्'? Observe सम् + अज + घञ् = समाजः 'society'; उदाजः 'driving out.' So also with अप्, as समजः 'a multitude or herd of animals &c., उदजः 'driving out of cattle.' See 3.3.69.
Why have we used वी with a long ई? For the sake of forms like प्रवीताः &c.
Vart:- Prohibition must also be stated (along with घञ् and अप्) of the affix क्यप्, as समज्या 'a meeting, fame.'
Vart:- The substitution is optional before ardhadhatukas beginning with a letter of वल् Pratyahara (all consonant except य) as प्रवेता or प्राजिता प्रवेतुम् or प्राजितुम्. See 6.2.144 for accent.,