How, according to Marx and Engels, does scientific socialism advocate for revolution without being utopian?

While the state machinery itself must be smashed and replaced, much of society in a young socialist state would be expected to resemble that of a liberal capitalist state (though that depends on how advanced the state was prior to the revolution, as well as its specific current challenges).

Many people look at, for example, contemporary Vietnam and say "how can this be socialist when capitalists remain infesting society?" They might even say that such a presence precludes it from being a "dictatorship of the proletariat".

Saying this, they fail to even consider how a government could be characterized as lead by the proletariat.

"Proletariat" does not simply mean "worker" or we would just use that word in all cases. The proletariat is a historically specific class that has not always existed and will not always exist. It is defined in opposition to the bourgeoisie, without whom it would not exist. Specifically, the proletarian is one who subsists by selling their labor (typically on a wage basis) to the private property owners (the bourgeoisie). The presence of the bourgeoisie is not only possible in but is in fact necessary for a dictatorship of the proletariat.

"But are we only talking about nomenclature? What about material reality and revolutionary strategy? What separates this from pseudo-semiotic bullshit?"

This is where utopianism comes back in. Building an entirely new economic model from scratch simply isn't viable on any level. The purpose of the dictatorship of the proletariat is to use markets to the advantage of the socialist project.

In all previous modes of government, the owning class has also controlled the state. This has the result of perpetuating class struggle, because the owning class is dependent on the labor value produced by the working class, and so must keep it in existence (as well as keep itself in existence).

The working class does not share perfectly symmetrical class antagonisms, because it is a very different class. Pertinently, it is not instrinsically dependent on the bourgeoisie's indefinite existence. Yes, from a vapid semiotic perspective there is no proletariat without a bourgeoisie, but the actual members of the proletariat as the dominated class do not have it in their material interest to be dominated indefinitely. Should a circumstance emerge where, due to technological innovation, it becomes more efficient for society overall to nationalize an industry, the proletariat will not hesitate to do so whereas the bourgeoisie certainly have.

Here's Marx and Engels discussing it in the Manifesto:

We have seen above, that the first step in the revolution by the working class is to raise the proletariat to the position of ruling class to win the battle of democracy. The proletariat will use its political supremacy to wrest, by degree, all capital from the bourgeoisie, to centralise all instruments of production in the hands of the State, i.e., of the proletariat organised as the ruling class; and to increase the total productive forces as rapidly as possible. Of course, in the beginning, this cannot be effected except by means of despotic inroads on the rights of property, and on the conditions of bourgeois production; by means of measures, therefore, which appear economically insufficient and untenable, but which, in the course of the movement, outstrip themselves, necessitate further inroads upon the old social order, and are unavoidable as a means of entirely revolutionising the mode of production.

In this way, under the DotP, the bourgeoisie are gradually nationalized out of existence as the country develops. This is the process Marxists refer to as the "withering away of the state". The predicted end result of this is a classless society, but that is in the distant future.

So the purpose of a revolution is, from a certain perspective, to enable a "withering away of the state" which is prevented when the bourgeoisie control the state.