To play devil’s advocate, none of those vanguard parties were ever allowed to exist peacefully. They were always attacked, from the inside and out, by capitalist and fascistic powers. It’s kind of hard to get rid of the state when it is needed to defend from other nations and groups looking to destroy it.

I’m not saying that a Vanguard party would necessarily ever voluntarily give up it’s powers and disintegrate into pure communism without a large part of the world struggling against it, but it would be more likely to.

That is just pure speculation, though, because we live in a world that has shown that it will struggle against communism until the end. The Vanguard Party idea is flawed, because it fails to account for this indefinitely long struggle, and fails time and time again to offer a valid exit strategy into the next stage of Socialism/Communism.