In the meantime, here is another post in the series on John Calvin and the parables:
It is well known that trees are sometimes preserved, not because their owners find them to be useful and productive, but because the careful and industrious husbandman makes every possible trial and experiment before he determines to remove them out of the field or vineyard. This teaches us that, when the Lord does not immediately take vengeance on the reprobate, but delays to punish them, there are the best reasons for his forbearance. Such considerations serve to restrain human rashness, that no man may dare to murmur against the supreme Judge of all, if He does not always execute his judgments in one uniform manner. A comparison is here drawn between the owner and the vine-dresser: not that God’s ministers go beyond him in gentleness and forbearance, but because the Lord not only prolongs the life of sinners, but likewise cultivates them in a variety of ways, that they may yield better fruit.