What an interesting question, one that gave me pause to stop and think. It is obvious that the produce from a vineyard is grapes, but what do “grapes” represent in this parable? The context in which Jesus delivers this parable is important. Mark 12:12 proves that Jesus’ parable was being made against the chief priests and scribes who wanted to silence Jesus and have him removed. In this parable God is the absentee landlord who first sends his servants (the prophets) to receive the landlord’s share of the fruit of the vineyard. When that fails, he sends his son, the rightful heir, but the tenant farmers kill him.
The vineyard and grapes are a metaphor for Israel:
For the vineyard of the Lord of hosts is the house of Israel and the men of Judah are his pleasant planting; he looked for justice, but behold, bloodshed; for righteousness, but behold, an outcry! (Isaiah 5:7 ESV)
“Israel is a luxuriant vine that yields its fruit.” (Hosea 10:1 ESV)
The produce from the vineyard is a metaphor for those who belong to God: Time and again Israel turns away from God and is disobedient yet He forgives His chosen people:
I planted you a choice vine (or fruitful vine in Septuagint), wholly of pure seed. How then have you turned degenerate and become a wild vine? (Jeremiah 2:21 ESV)
God ejects the wicked tenants and gives the vineyard to others: The wicked tenants want to kill the son, the rightful heir, but when the owner of the vineyard returns there will be righteous retribution:
He will put those wretches to a miserable death and let out the vineyard to other tenants who will give him the fruits in their seasons. (Matthew 21:41 ESV)
The Kingdom of God will be taken away from those who reject the Son: Jesus quotes from Psalm 118:22-23 (about the cornerstone rejected by the builders) and issues this warning to the chief priests and the Pharisees, the religious leaders:
Therefore I tell you, the kingdom of God will be taken away from you and given to a people producing its fruits. And the one who falls on this stone will be broken to pieces, and when it falls on anyone, it will crush him. (Matthew 21:43-44 ESV)
The produce of the vineyard are those who belong to Jesus, the Son and heir, and who bear fruit:
”I am the true vine, and my Father is the vinedresser. Every branch in me that does not bear fruit he takes away... he prunes that it may bear more fruit.” (John 15:1 ESV)
Conclusion: The produce of God’s vineyard are all who accept the Son of God and who abide in Him.
It is worthwhile to read all three accounts of this parable:
Matthew 21:33-46; Mark 12:1-12; Luke 20:9-19
P.S. Please be aware there was only one answer to this question on Wednesday evening when I started to do some research. I was unable to post this answer till Thursday morning and had not seen the other answers.
EDIT Regarding the quote from Psalm 118:22-23, this snippet of information landed in my in-box this morning. It's about people who don't believe Jesus is the Messiah:
In 1851, the Truro Church of England minister William Haslam was preaching from his pulpit on the text in Matthew 22:42, “What think ye of Christ?” This was Jesus’ question to the hypocritical Pharisees who refused to believe he was the foretold Messiah. As he preached, he became convicted that he was just as hypocritical as those Pharisees, because he did not believe Jesus to be the Messiah, the Son of God. He broke down and wept, and was converted to Christ. A man stood up in the congregation and exclaimed, “The parson’s been converted! Hallelujah!” and the rest burst out in praise. After the commotion died down, others testified that they, too, had been converted, such was the convicting power of the Holy Spirit during that half-finished sermon.