Vonnegut wasn't a Too Good For This recluse like Pynchon or Salinger or whoever, and the content of his books made him fairly well insulated against charges of being a sellout; they probably offered him a bunch of money and he shrugged and said sure.
It was the sellout part that surprised me. I just wouldn't ever expect him to advertise anything. But I suppose as cynical as he was, he just took the money and agreed.
I do not, but I do know that there's a "coffee achiever" line in Weird Al's Dare to Be Stupid, which is how I tracked this down. And then saw Vonnegut and was like WTF? And then thought WTF about this entire commercial. Did America even need to be sold on coffee?
I'm not so old I can say anything about the 80s, but in my experience people did not drink anywhere near as much coffee before Starbucks took off. Seattle was considered weird for after-morning coffee consumption, and even wake up drink was more often tea.