Vedi stands third, writes v, and carries the numeral 2. Its name is the Slavonic verb form meaning I know.
Its shape is Greek beta and its number is the number of Greek beta, and both were taken over without adjustment. The reason it writes v rather than b is that by the ninth century Greek itself had stopped pronouncing beta as b. Byzantine Greek had shifted the sound to v, and a Slav learning to read Greek in Thessalonica or Constantinople would have been taught that this letter says v. When the sound values of the Greek alphabet were mapped onto Slavonic, that teaching came with them.
The consequence for the alphabet is one of the most instructive facts about it. Cyrillic does not record the Greek of the classical grammarians; it records the Greek that was actually being spoken and read aloud in the Byzantine church at the moment the mapping was made. The same shift explains why izhe, whose shape is Greek eta, writes i rather than a long e, and why izhitsa, whose shape is Greek upsilon, is read as i in most positions.
An alphabet borrowed from a living tradition preserves that tradition at a date. Read carefully, the Cyrillic sound values are a small piece of evidence for the pronunciation of Greek in the ninth century, independent of anything the Greek grammarians wrote down.