Sometimes Vader sounds a bit impatient - it’s kind of funny when some Officer has to bother him when he’s inside that meditation chamber, and Vader is like “What is it, General?” You can hear slight impatience in his voice. But he never rages or loses his temper. He’s the complete opposite of Kylo Ren. And definitely way more subdued than Anakin, who was constantly flipping out in both Episode 2 and 3. We can make excuses for this personality difference by saying that over the years Vader became more mature as a Sith and learned to channel his anger in more constructive ways or whatever, but honestly the reality is that Anakin’s portrayal just kind of sucks.
What makes you say that last line? Why wouldn’t ruling the galaxy for 20 years, having settled into your choice to take wrong path lead to a less “flipping out” personality? The “flipping out” was all due to his internal struggle between his better angels (Obi Wan) and the devil in his ear/his baser desires (Palpatine). The struggle is over after he ends the Jedi and loses Padme. He has nothing left but his delusions about bringing order to the galaxy and overthrowing Palpatine (both of which he tries to sell Luke on, and neither of which involve turning away from the Sith path).
One telling moment for me is when Anakin tries to “reason” with Obi Wan one last time, and Obi Wan just tells him he is lost. When Anakin says “This is the end for you, my master” that is pure Vader. Monotone, emotions tamped down, the whole bit. My point being, Vader is the side of Anakin that lives in denial. Anakin starts to come back in Return of The Jedi, and for my money, the man that Luke starts getting through to acts exactly like Hayden.