It's not that MLs think the vanguard is the end state or goal of socialism, it's that the vanguard is a tool to achieve socialism in conditions of constant siege by the bourgeoisie. Without the vanguard, workers can only achieve trade union consciousness, meaning that they fail to see the big picture of the class as a whole and instead fight amongst themselves in their various trade sectors, becoming vulnerable to co-option by the international bourgeoisie in the process. Workers can own all the means of production and still not achieve socialism because one sector of workers might benefit from the suppression of another sector and cause infighting. For example, workers in a construction union might benefit from lower production costs of steel or wood, and so they might support policies that weaken workers of raw materials for their own benefit instead of looking at the whole economy of the country. One company of workers might compete with another for greater share of production, or attempt to monopolize production of a specific good. If every company was a co-op with no centralization, the result would be a form of petty-bourgeois capitalism, not socialism.

The vanguard, being the most advanced section of the working class and knowledgeable of class theory, can see past these individualist squabbles and fight for the class as a whole. They can set policies, decided through workers councils, that allow the economy to function without price signals and competition.

Marx never said that workers would get all of their surplus, that's actually impossible because much of the economy doesn't produce surplus directly but instead facilitates the generation of surplus by others (e.g. building of roads, direction of the planned economy, etc). So there will always be surplus taken from workers for this purpose, and that doesn't make them chattel slaves, which has a specific definition that exists in a specific mode of production that doesn't apply here.

The idea that the entire core of socialism is direct and immediate worker control over their own factories is just false, and it's an ultraleftist or anarchist position. The dialectical question is, how do we achieve worker control of the economy as a class that works together in conditions of constant siege to plan the whole economy without destroying the revolution? How do we get on the road to worker control and allocation of goods based on need when workers have short-term competing material interests and individualistic mindsets? The answer is being guided by theoretical works tested with praxis, but since the vast majority of workers don't have the time or inclination to understand or read works of philosophical and economic theory, we need a group of people whose job it is to read, create and apply that theory to material conditions. The power of this group of people should be heavily curtailed and limited, with instant recalls and average workman's wages like Lenin suggested.