Q:

Why is Darth Vader such a brute compared to Anakin?

Darth Vader is often described as a tool "to be wielded rather than to wield", a brute who only knows how to solve everything with a lightsaber. Anakin Skywalker, meanwhile, is a cunning warrior renowned on both sides of the Clone Wars for his unconventional ways, snatching many a victory against the odds by out-smarting the enemy.
What changed? Did he choose to stop using his smarts by choice? Or did the fires of Mustafar fry his brain?

A:

The key trait that distinguishes Darth Vader from Anakin is anger. Darth Vader may have a cool exterior, so he is not actively angry; but Anakin's character is consumed by anger, and therefore forgoes an intellectual approach.
This isn't limited to Star Wars, it is a basic form of human behavior. People who are driven by anger have little patience for diplomacy or subtle behavior. They usually even have issues with lowering their voice or adapting a non-threatening body posture.
We use phrases like "consumed by rage", which inherently describe how someone loses part of their personality, and is driven by much more instinctive (and less intellectual) behavior.

How anger makes you think differently
The researchers had participants vividly recall either an experience that either made them really angry or really sad, and then exposed them to an essay arguing in favor of raising the legal driving age from 16 to 18. The essay that participants read were attributed either to "a group of transportation policy experts at Princeton University" or to "a group of students at Sinclair Community College in New Jersey." In reality, the essays that the participants read were exactly the same, so only the supposed author (the "source") of the message differed. Afterward, all participants were asked how persuaded they had been by the arguments. Now, whereas the sad participants did not differ in how persuaded they were depending on the message source, the angry participants were significantly more persuaded if the (same) message was attributed to the Princeton policy experts.
[..] If participants engaged in heuristic information processing, by contrast, they should rely more on surface-level features of the message-- such as the credibility of the source. The results clearly showed that the angry participants were relying more on the heuristic features of the message, since they found the exact same message more persuasive based on who the message was attributed to.

In other words: angry people draw shorthanded conclusions. Which is exactly why Darth Vader, "angry Anakin", has lost the capability of being clever and cunning.
There are many cases of this, I'll list a few off the top of my head:

In The Force Awakens, Kylo Ren starts smashing a computer with his lightsaber when he got angry. Intellectually, destroying a machine only leads to more work (fixing it), but he did it anyway, because he was angry.
In Attack on Titan, Eren Jaeger, the protagonist, has a clear anger problem. He means well, but his reactions to injustice are filled with so much anger that he is incapable of finding a diplomatic solution (or even just lowering his voice)
In Deadwood, Seth Bullock looks like the "good guy" of the show. However, once you dig a little bit deeper, you notice that he has a severe anger management problem. Initially, this is considered a lack of patience when dealing with criminals (which isn't too bad, as he is a sheriff by nature). But as the show progresses, he clearly acts out of line beyond what can be considered morally correct.
In A Few Good Men, Col Nathan Jessup ends up publically declaring his responsibility; which he had been keeping a secret up until then. The only reason that he ends up spilling the beans is because Tom Cruise's character gets him angry enough that he forgoes a subtle approach and instead grandstands about why he thinks is right.
This actually applies to many court cases where the bad guy ends up yelling that they did it. Almost always, the opposing lawyer makes them angry enough that they can't think clearly anymore and instead make their true feelings clear. Another instance of this would be Charles McGill in S03E05 of Better Call Saul.
In Doctor Who, the 11th Doctor (Matt Smith) talks about his previous incarnation (David Tennant). He speaks negatively of him, because he was driven by his emotions (anger being one of the major ones). Although he does not think what he did was particularly wrong, he does reveal that he's glad to let go of those emotions as they clouded his judgment. (The same 11th Doctor then later falls into the same trap, militarizing a large part of the characters because Amy is threatened. Again, the Doctor needs to let go of his anger in order to be levelheaded).

I'm sure that this list could go on for ages if I keep thinking of more examples.

