If Rome wants to build a major theology on a similtude that does not correspond to the Last Supper, that is fine, but they need to be honest about other similtudes Jesus spoke as well. He is the Door; the Vine; the Word. We understand what Jesus meant by "the Door." He is the door to the Father. We don't have an earthly door in which to enter, because it is in the Person of Jesus Christ. We understand the term “Word” (Greek logos) designates God the Son with respect to His deity; “Jesus” and “Christ” refer to His incarnation and saving work. We do not "eat" a Bible to receive the Word. The Vine is used to show The Father is in the Son, the Son in the Father, The believer in the Son and the Son in the believer. Quite a 'complicated' circuit thus the Vine.
These are powerful similtudes, pictorals if you will of a definate REALITY of Jesus Christ. None of them are giving us a physical reality other than the death and Resurrection of Jesus Christ. That happened once for all.
We are told by the Master in verse 63:
63 It is the Spirit who gives life; the flesh profits nothing; the words that I have spoken to you are spirit and are life.