Physically Wlad was in his prime, he's sharpened up his defense and takes less risks I agree with that much. In a way its similar to Lennox but we don't know if Wlad ever beats Sanders, Lennox was looking to take McCall out and got caught out doing so, Wlad just got bumrushed so we don't know if he'd ever beat sanders. He still looked fragile against Peter and scared against none punchers. You could make a case Wlad was better then as he bigger balls. Lennox never became as negative/gun shy as Wlad has.
It all boils down to excuse-making for losses. And other such nonsense. In most cases it should be obvious when a heavyweight is 25-28, has proven himself enough to be rated at or near the top of the division, then we know he's in his prime. Lennox Lewis was in his prime from about 1991 or 1992 onwards. Until 2001 or 2002 really. Just because the American press exaggerated his shortcomings c.1993 and perhaps exaggerated his abilities c.2000 doesn't have any bearing on it. Sure, he might have improved a bit with experience, dedication and good coaching, but he was near-enough the same fighter. And being human, his performances varied from one fight to the next. Excuse a fighter for losing pre-prime, when he really was a raw or inexperienced up-and-comer, but not a guy who has several victories over genuine contenders.
To me, "prime" means being well developed, or anything near it. For instance, i think Tyson's prime lasted from 86 to 97. Many will argue that he was past his prime against Holyfield. Certainly he was past his peak, but he was still an awesome puncher who disposed of Bruno faster than anyone ever did. Which answers the topic: yes, Wlad was in his prime.
I would say Vlad was slightly pre-prime for Sanders. Even thought he was physically at his peak, Vlad's tactics still hadn't been rounded out and improved to the degree that he is at now. More than anything else, though, I think Vlad and Sanders would always have matched up stylistically and physically that are dangerous for Vlad.
It happened to Lewis again with McCall and then Peter but an in shape Lewis also showed a pair of wiskers and a carefull style in dangerous fights (Tua and Tyson) Vlad has been fighting carefull. I think the changes Stewart made were alot more sparring with the men to make them comforable in the ring and to fight tall and mantain leverage.. He did it with Hearns, Lewis and Vlad...Can it happen to Vlad again...Sure but he has improved with caution and too many fighters are reluctant to wade in to his arsenal and that will keep him on top for a while...Povetkin if he can take it has a style of pressure but has not impressed me lately but his choice of opponents and there styles has been poor
He was in his prime, or very close to it. Sanders simply beat him to the punch. At the risk of sounding biased, I'm not convinced the Wlad of now would fare that much better.
Sanders won fair in square. Wlad underestimated Sanders, and was completely unprepared for what he could bring into the ring. Upsets like this happen from time to time in the heavyweight divison...espcially when the opponent is a puncher with a style advantage, You could say Wlad was in his physical prime for that match, but he was far from his mental prime, and still lacking some world class experience. Wlad worked with Steward ( who was not in his corner for Sanders ) and mastered many of the finer points of offense, defense, and ring generalship. The current version of Wlad from Brewster II to present is much better vs. southpaws, and would have beaten likely beaten the version of Sanders who defeated him. In hindsight, the defeat was good for Wlad's long term career. Wlad learned some of his deficiencies as a fighter, and built a new game plan that makes it extremely difficult for the other guy to take advantage of his weakness.
I think that Wlad's loss to Sanders was probably a variety of different factors, ranging anywhere from complacency to stylistic issues. I'm not sure that the whole " work in progress " argument, really applies here. He was 27 years of age, a veteran of over 40 professional bouts, and an olympic gold medalist with a rather extensive amateur career. He was also in superb condition and had some noticable physical advantages over Sanders. I think its true that some of his skills did improve under the instruction of Manny Steward, but I generally think that he was in his prime.