I would argue that Vaders turn to the dark side started when he was alittle kid. Lucas cut a scene from Esisode I where the kid gets into a fight with a young Greedo. Greedo was apparently claiming that Anikan cheated in the Pod Race. Then Anikan flashes Mace Windu a pretty evil look when the Jedi Council declares that he is too old and will not be a Jedi. Of course I am only half joking.
I really don't think that the turn was that quick. The Emperor was working on Anikan for many years. Playing the Jedi council against the Senator's council and friendship with him. Telling Anikan that he will be the most powerful of Jedi. Boosting his ego.
Plus I think that Anikan was created by the dark side of the force. So he already had it in him. :)
Yeah, if you can get past all the sucky dialogue, the plot is excellent. All the things Anakin did that Yoda told him not to do came back and bit him on the butt in the end. Sidious used all of them...as he used all the weaknesses existing in the Republic...in grand Machiavellian manner to his own benefit in his master plan to grab power.
And the fault doesn't like entirely with Anakin. As Yoda said in Episode I, the Jedi were becoming blind, complacent, and overconfident, unaware of the growing threat of the Dark Side. Their inaction (i.e. when good men do nothing) allowed evil to rise.
If only Lucas could swallow his control freak ego and let someone else write and direct. These three movies would have been the classic tragedy, full of intrigue and subplot.