What exactly is "vulgar materialism?" Why is it incorrect?

Think clockwork. We used to believe that animals couldn't feel pain. They weren't complex subjects situated in a complex environment, but simple biological machines that would yelp in response to a stimuli. It was reductive and lost sight of the bigger picture. Animal cognition is the actual materialist understanding which then unlocks all of the other interdisciplinary scientific observations, its materialistic truth confirmed through those. We do vulgar materialism when we do something like MAGA Communism's class reductionism. Throw out all of the superstructural issues that people experience the economy through in favour of a purely economic critique that abstracts them into one generic class. Sure you're left with an observable number and you've stripped away everything that isn't quantifiable, but your materialist critique is limited and acting on it would lead to something like the chauvinism that limited American communists in the 1930s.

It just means an analysis that's not sufficiently dialectical. They neglect the dialectical part of dialectical materialism. In terms of politics, they fall for the exact opposite of Great Man Theory, the exact opposite being the idea that humans are completely powerless against the force of history and that humanity is destined to be cast adrift against structural forces beyond their control. They take that quote from Marx about man not making history of his own choosing and erroneously invert it to say that history makes man. They neglect the fact that history is ultimately made through human effort, which means history is at least forged through human will, sacrifice, and ingenuity.

Humanity itself is guided by its consciousness. After all, the working class will not liberated itself until it recognized that it itself constitutes a class and that it must strive to act within its own class interest. Revolutionaries are supposed to awaken the spark within the working masses so that its consciousness changes and workers go from a class in itself to a class for itself. And through their qualitative shift in consciousness, the working class can embrace a liberatory politics and seize control of their own collective destinies through the overthrow of the bourgeoisie. The drivers of history will now drive history entirely on their own terms.

My understanding was it's really just boiling things down to two classes butting heads, while failing to account for race, gender, and various cultural elements that when failed to account for really leave folks in the dust.

Great answers in this thread already so I'll talk more generally:

When we try to understand the world by fitting it into conceptual boxes, we are necessarily reducing it to simpler, more digestible models. This process is a double edged sword in that it allows us to understand and communicate ideas about something that's otherwise infinitely complex, allows us to brush over a million other variables so we can focus on key ones of interest. But at the same time these models are not reality, variables are being ignored or de-emphasized, leading to potential inaccuracy (rather than merely imprecisions). Additionally, that human component is prone to being influenced by bias/ideology.

So in the multitude of concepts clustered around the word "materialism", some of them may ignore or de-emphasize variables that actually have meaningful influences, resulting in models that are too reductive and that might lead one to make choices that don't have the desired and expected outcomes.

Class reductionism is one you'll commonly read about in Marxist circles. Mechanistic materialism (as opposed to dialectical materialism) might be another. But as with most categories, the lines are fuzzy and sometimes arbitrary. Two people who ascribe to materialism might call each other vulgar materialists because they disagree on which variables to de-emphasize or where to draw the line between idea and material. Similarly among diamats and what constitutes base versus superstructure.

i might be dumb (philosopher) but i see it as the difference between claiming that consciousness is entirely nonexistent (techbro death cultist 'meat computer' shit) vs. claiming that consciousness is in a dialectical relationship with physical reality (whereas Idealism claims that physical reality is entirely nonexistent and 'vulgar materialism' claims that consciousness is entirely nonexistent)