I'm currently reading about the Renaissance and this is a picture of (according to my book) the first Fauxbordon composition. But the thing I'm actually wondering about is: what does vos mean? I thought of something like "melody/main voice" but I can't find anything.
As replete says in the comments, "vos" is actually just the first word of this part of this mass: listen here.
In the context of a mass, we call this type of opening the intonation:
Intonation. The first pitches of a psalm tone or other form of plainchant, which have the function of establishing the correct pitch for what is to follow and are thus often sung by soloists in preparation for the entrance of the choir. See also Intonatione.
(Source: The Harvard Concise Dictionary of Music)
The word vos in latin and some other latin-based languages means : you.
This may simply be the way the sheet music was attempting to indicate that this is the part that "you" sing, while the other parts are assigned to others fauxbourdon and tenor.
In other words, if you are not singing the other parts then this is your part. However, I do see how that seems simplistic and this is a bit ambiguous.