From 1990, this very cool Vonnegut article sums up how he felt about Heinlein.
From the article – a Heinlein quote about Stranger:
I was not giving answers. I was trying to shake the reader loose from some preconceptions and induce him to think for himself, along new and fresh lines. In consequence, each reader gets something different out of that book because he himself supplies the answers . . . . It is an invitation to think — not to believe.