Three of the four readings for Sunday (Old Testament, Psalm, and Gospel) are full of stuff about grapes and vineyards. To make sense of the imagery, keep in mind what a vineyard is - a business that requires a significant financial investment, plus a lot of physical labor, takes years before it turns a profit, and produces something so valuable that it has to be protected from thieves.
The heart of both vineyard stories (the one told by Isaiah in the Old Testament and the one told by Jesus in the Gospel) is that they are dire warnings to the people to whom the stories were being told. The warning was given, because the people (in both stories) were separating themselves from God, ignoring and misusing the gifts that God had given them.
In Jesus’ time, people were working hard to ignore the warning sign given by Isaiah’s story.
Which is why Jesus retold it, to make the warning even clearer. When you hear Sunday’s readings keep in mind that warning, and how easy it can be to ignore the warning sign.
Sunday’s Readings: http://www.usccb.org/bible/readings/100514.cfm