Being Lawful does not mean you will never act Chaotic. Being good does not mean you can never do evil acts.
In other words, no one is perfect, everyone makes mistakes.
I say Darth Vader is Neutral Evil. He is all about what he wants, not what Padme wants, not what his son wants, not even about what his mother wanted. He killed many Jedi, and Jedi children because what what he wanted was what was most important to him.
He served those who gave him what he wanted, even if it was just to stay alive. Which is why he served the emperor, but desired to overthrow him.
He wasn't very Chaotic, because he tolerated the yoke of the Emperor for a very long time.
His redemption was significant because he finally put the life of his son before his own. The first seriously unselfish act he ever did. Every other unselfish act was to pay a price to get something he wanted more, such as to be trained in the ways of the force.
He did things that were chaotic, he did things that were good, he did things that were lawful, he did things that were horribly evil, but they were all done to get him what he wanted. Neutral Evil, overall.