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).
You’re not really disagreeing with me here. You seem to agree that Anakin does flip out more than Vader, but you provide an explanation for why you believe this observed difference in behavior is plausible.
All I can say is that your explanation (essentially saying Vader had no reason to flip out once he settled into his role as a powerful Sith) doesn’t really help make Anakin and Vader seem like the same person to me. I mean ultimately this is subjective, but I just can’t really picture Darth Vader saying a lot of the stuff Anakin says with the cadence or wording that Anakin uses. As one example, I can’t imagine Vader saying “Liar!! You’re with him! You brought him here to kill me!” Instead, Vader would just say “You have betrayed me. You have brought Obi Wan to kill me.”, while pointing his finger in Padme’s face.