Metaphors are by their very nature open to contradiction by other equally valid and applicable metaphors. E.g. Jesus is "the Door" (John 10:7) as well as "the Bread" (John 6:35) of heaven. Quite clearly, these are contradictory if taken literally (i.e the opposite sense in which metaphors are to be taken), but not when recognized as the metaphors they are: Jesus is the Door, because He is the sole entrance to heaven, just as a door is the sole entrance to the house (John 14:6); Jesus is the Bread, because He "gives life" (John 5:21; 6:57b; 15:5 etc.); but doors don't give life, nor does bread serve as an entrance to the house. Again, Jesus is said to be "the Word" (John 1:1; Revelation 19:13) of God, even though God doesn't have a tongue; and the "Son of God" (1 John 4:15; Revelation 2:18; John 10:36) even though God doesn't have sexual organs.
Therefore, Jesus can be the offshoot or branch of David, seeing as He is a significant descendant of David, namely, the Messiah, who comes from David, and 'branches out' into the whole world (Daniel 2; 7; Mark 16:15), as well as the Vine, since the vine is the source of life for the branches which live on it, and the branches are dead if not for the life-giving nutrients provided by the vine, just as Christians cannot be born again or live a Christian life, since, "apart from [Christ they] can do nothing" (John 15:5).
Also, Christ, being both God and man (John 1:1-14; Revelation 1:17) can be spoken of in either his capacity as man, or as God, being both at once, and as such we find things contradictory to the nature of God, and to the nature of man, both said of Christ, because He is one person only, but has two natures of which these things are properly said respectively:
And I looked for the voice that spoke to me. And turning, I saw seven golden lampstands; and in the midst of the seven golden candlesticks, one like the son of man, clothed with a garment coming down to the feet, and having a golden sash. And his head and his hair were white - as white wool, and as snow - and his eyes were like a fiery flame, and his feet were like fine brass when it glows a furnace; and his voice like the sound of many waters. And he had in his right hand seven stars. And from his mouth proceeded a sharp sword: and his face shone as the the sun when it shine in its vigor. And when I saw him, I fell at his feet as though dead. But he laid his right hand upon me, saying: Fear not. I am the First and the Last, the Living One; who died; and behold I live for ever, and I have the keys of Death and Hell.