Vede writes the consonant v and takes the numeral three. Its name is a verb form rather than a noun, the first person singular meaning I know, and it is the second word of the sentence that the opening letters of the alphabet spell out when their names are read in order.
The sound it records is the one Greek beta had come to make by the time of the mission, and that is a large part of why the Glagolitic order looks the way it does. A ninth century Byzantine reading a Greek text pronounced beta as v, so when the alphabet was arranged the letter answering to beta was given the Slavonic v, and the plain b had to be provided separately. The two letters therefore sit side by side in the Slavonic order in the reverse of the sequence a classicist would expect, with b before v, and the numeral values follow the Slavonic arrangement rather than the Greek one.
In Old Church Slavonic v is common as a preposition on its own, written as a single letter attached to the word it governs, and manuscripts show it running together with what follows in a way that can make word division difficult for a modern editor. The sound has been stable across the Slavic languages, and where an early text writes vede a modern Slavic language will still have v, so the letter offers none of the dialect evidence that yat or the nasal vowels do.