I found it. It was by Vonnegut indeed.
"The Euphio Question by Kurt Vonnegut"
It begins with :
"Lew, Fred, and I found peace of mind by sitting in easy chairs and turning on a gadget the size of a table-model television set. No herbs, no golden rule, no muscle control, no sticking our noses in other people's troubles to forget our own; no hobbies, Taoism[I1] , push-ups or contemplation of a lotus. The gadget is, I think, what a lot of people vaguely foresaw as the crowning achievement of civilization: an electronic something-or-other, cheap, easily mass-produced, that can, at the flick of a switch, provide tranquillity. I see you have one here."
and ends with
"In closing, I'd like to point out that Lew Harrison, the would-be
czar of euphio, is an unscrupulous person, unworthy of public trust.
It wouldn't surprise me, for instance, if he had set the clockwork on
this sample euphio set so that its radiations would addle your
judgments when you are trying to make a decision. In fact, it seems to
be whirring suspiciously at this very moment, and I'm so happy I could
cry. I've got the swellest little kid and the swellest bunch of
friends and the swellest old wife in the world. And good old Lew
Harrison is the salt of the earth, believe me. I sure wish him a lot
of good luck with his new enterprise."