Jesus had just said to His disciples, “Get up, let us go from here” in the last verse of John 14, so perhaps it was passing some grapevines on the way to Gethsemane which occasioned Jesus to talk about how “I am the true Vine, and My Father is the vinedresser” in John 15:1.
We see here an example how Jesus used whatever was at hand to teach truth to His disciples. On other occasions He might refer to “the birds of the air” — possibly flying by at that moment (Matthew 6:26) or teach a lesson from a barren fig tree (Matthew 21:18+). These remind us that God has placed object lessons all around us in His Creation, which will point us to Himself and to spiritual truths (Psalm 19, Romans 1, etc.). They are there to help instruct us — if we will be observant enough to see them, and meditate on the truth they contain, and the One to whom they are pointing:
— Consider a vineyard and learn: “He is the Vine; I am a branch. I must abide in Him continually or I can do nothing.”
— Gaze at the sky and say: “The heavens are telling the glory of God, and their expanse is declaring the work of His hands.”
As Romans 1:20 clearly states: “For since the creation of the world His invisible attributes, His eternal power and Divine nature, have been clearly seen, being understood through what has been made.”
In other words, there are lessons God has placed all around us in His creation — if we will only take the time to see them!