I think there is a lot of historical evidence that dismisses your claims. Polyamory, and/or communal parenting, has existed in many forms amongst many different indigenous peoples, and it is still practiced today. There was a time in our past where children being raised by many different parents was the ‘norm’, and an argument could be made that it is a more natural form of child rearing than our ‘norm’ of monogamous parent couples.
There is no evidence of people that practice communal or group parenting having issues with violence or jealousy, that is just your assumption. There is evidence that these kind of situations could be advantageous. The child has more people to pay attention to them and can feel a better sense of community. They are also being socialized better and are being shown a wider variety of perspectives, etc.
I would definitely suggest you look into it for yourself, if you are curious why your assumption is wrong.
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.