You see, that's were at I see the contradiction.
You can either have the proletariat as the "ruling" or self-determined class or a Vanguard Party, determining the fate of the rest of the proletariat.
You say they would be the "most class-conscious workers", but who is to determine that?
And was this the case in the Soviet Union?
Imo, during the Russian revolution, the Bolsheviks simply claimed and executed their power and accused their comrades who dared to question this of being "counter-revolutionary".
However. Maybe I understand the concept of a Vanguard party wrong. But do you think that such Vanguard party should come with a tight system of reassuring their actions and the people "in charge" with the working masses, in order to keep probable power hungry individuals from said power?
Kinda like Mao's mass-line, but without Mao as an unchallenged leader and without straight-up executing people.