“It should be mentioned that any external supernatural agency plays no part whatever in the moulding of the character of a Buddhist. In Buddhism there is no one to reward or punish. Pain or happiness are the inevitable results of one's actions. The question of incurring the pleasure or displeasure of a God does not enter the mind of a Buddhist. Neither hope of reward nor fear of punishment acts as an incentive to him to do good or to refrain from evil.”

"The God-Idea"
What Buddhists Believe (1993)

Source: Guide for the Perplexed (c. 1190), Part III, Ch.24

In A Man Without a Country (2005) p. 80–81 Vonnegut makes a very similar statement:
God Bless You, Dr. Kevorkian (1999)
Context: About belief or lack of belief in an afterlife: Some of you may know that I am neither Christian nor Jewish nor Buddhist, nor a conventionally religious person of any sort.
I am a humanist, which means, in part, that I have tried to behave decently without any expectation of rewards or punishments after I'm dead. My German-American ancestors, the earliest of whom settled in our Middle West about the time of our Civil War, called themselves "Freethinkers," which is the same sort of thing. My great grandfather Clemens Vonnegut wrote, for example, "If what Jesus said was good, what can it matter whether he was God or not?"
I myself have written, "If it weren't for the message of mercy and pity in Jesus' Sermon on the Mount, I wouldn't want to be a human being. I would just as soon be a rattlesnake."

Source: The Sparrow

Letter X
The Nemesis of Faith (1849)
Context: I will be candid. I believe God is a just God, rewarding and punishing us exactly as we act well or ill. I believe that such reward and punishment follow necessarily from His will as revealed in natural law, as well as in the Bible. I believe that as the highest justice is the highest mercy, so He is a merciful God. That the guilty should suffer the measure of penalty which their guilt has incurred, is justice. What we call mercy is not the remission of this, but rather the remission of the extremity of the sentence attached to the act, when we find something in the nature of the causes which led to the act which lightens the moral guilt of the agent. That each should have his exact due is Just — is the best for himself. That the consequence of his guilt should he transferred from him to one who is innocent (although that innocent one he himself willing to accept it), whatever else it be, is not justice. We are mocking the word when we call it such. If I am to use the word justice in any sense at all which human feeling attaches to it, then to permit such transfer is but infinitely deepening the wrong, and seconding the first fault by greater injustice. I am speaking only of the doctrine of the atonement in its human aspect, and as we are to learn anything from it of the divine nature or of human duty.

Song lyrics, Mutiny (1993), Mutiny in Heaven