What does it mean vaporized mean in the book 1984?

To be vaporized in 1984 means to be treated as if you had never been alive, no matter how important you might once have been. It means being regarded as if you had never been born. Winston thinks about this as he contemplates the woman in the next cubicle:

He knew that in the cubicle next to him the little woman with sandy hair toiled day in day out, simply at tracking down and deleting from the Press the names of people who had been vaporized and were therefore considered never to have existed.

Vaporization is a means of social control in Oceania. Anyone who could conceivably harbor an independent thought is in line for being vaporized. Winston learns this is deliberate policy when O'Brien gives him the secret book called The Theory and Practice of Oligarchic Collectivism by Emmanuel Goldstein. In it, Winston reads the following:

vaporizations are not inflicted as punishment for crimes which have actually been committed, but are merely the wiping-out of persons who might perhaps commit a crime at some time in the future.

Before he is arrested, Winston calculates which people he knows will inevitably be vaporized and puts himself on the list. He later comes to understand that the vaporizing is part of the Party's attempt to control all of reality. By wiping out all records and traces that someone existed, the Party can control history. In a sense, that person never did exist, because no memory remains of his or her existence. This ability to will into being its own reality is the kind of power the Party craves.