Actually, he started working on Djem So/Shien right after AOTC, and was pretty damn proficient by ROTS. However, with the limited mobility and the inconvenience of using both hands (Keep in mind he can't do certain maneuvers with those arms because of his outfit; he also can't twist to the side, roll things over his wrist, etc.), Vader improvised the style. It's still reliant on power and aggressiveness, but he uses it mostly one-handed. Watch ESB and you can see.
If I had to answer the question literally, I'd say he did not master the form; he basically went into a custom form right after he fell to the dark side (Which is a trend).