/w/ > /v/ in Latin have
>For that, we need a substrate which has /v/, but no /w/. Greek
>fulfills that criterion
Modern Greek does.
Ask yourself at what time Greek /v/ developed. By the early empire, written
<b> was pronounced as a bilabial fricative (attested, for example, by the
spelling of Vespasian, already quoted in this thread, and see Horrocks
"Greek: a history of the language and its speakers" p112 section 6.4 (19)
(ii).) The rise of spoken /v/ from this, and from written upsilon between
vowels and before voiced stops, is considerably later. Probably too late to
affect Proto-Romance.
Peter