This is a very interesting idea to think about, and you might benefit from moving this question to the discussion board of this group where you would get many more responses. However, just initially, Winston Smith is not vapourised as we assume Syme is. Smith is broken, completely and utterly, by O'Brien.
Let us remember though when we consider this question that the model for this world, Stalin's Russia, was one that was famed for its purges. A key aspect of this was the way that those involved in leading the purges (which were responsible for the deaths of millions of Russians) were themselves victims of later purges. Stalin was notoriously paranoid, which could perhaps explain this. However, the society in 1984 is one which denies safety to all of its citizens, and therefore I think you are right in thinking that O'Brien at least has the potential to be purged later on in the world of this novel.