Jesus spoke of Himself as the vine, the true vine, because He is literally a spiritual vine:
"I am the true vine, and my Father is the gardener." - John 15:1.
I am of the view that physical vines were created by God to reflect who God is through Christ as the true vine, and our knowledge of physical vines helps us to understand and appreciate who He is as the true vine. This is why we are told:
"For since the creation of the world God's invisible qualities His eternal power and divine nature have been clearly seen, being understood from what has been made" - Rom 1:20
The invisible qualities of God through Christ as the true vine are clearly seen, being understood from what we know about physical vines. Jesus as the true vine is not a mere metaphor; He is indeed the true vine.