> it's not in my Liber, therefore it wasn't in the "Old rite" presumably

Beware of that reasoning. Many chants not found in the LU did belong to the «Old rite» (as you call it) and could be found in the Graduale Romanum, the Antiphonale Romanum, some other unofficial chant book, or were not yet published.

This particular communion, however, is indeed missing from the 1962 Missale Romanum.

> BCKS

That's the list of some ancient neumeless medieval manuscripts (giving only the words of the chants) where this particular chant occurs. (I don't have my GT here, so I can't tell you which mss. are those.) I guess «Sext.» is a reference to the Antiphonale Missarum Sextuplex (prepared by René-Jean Hesbert), that lists the earliest sources of (nearly!) all the chants for the Mass of the Gregorian repertory (and that attributed those letters to the manuscripts in question).

> Was "vitis" composed recently?

In face of the above, no.

It is so interesting that they are so similar. It sort of causes one to do a double take, like liturgical deja vu.

I believe only the first member (until «palmites» or «alleluia») is similar, and the recourse to phrase migration from one chant to another is something not unheard of in Communions. Furthermore, while «Ego sum vitis» is classified as a plagal Tetrardus, this member sounds like mode IV transposed (I will gladly stand corrected if I'm wrong), and the chant keeps sounding like that nearly until the end; only at the very last moment will it switch to mode VIII. Concerning that, Apel says that «The most remarkable aspect of the Communions is their tonal behavior. No other type of chant includes such a large percentage of melodies showing tonal instability and, as a result, ambiguity of modal assignment.» (p. 312)