Ranieri was considerably better then Vialli for Chelsea. And he in general is a better manager without any doubt, you couldnt be any more biased. Vialli couldnt do squat in lower league team like Watford and no one wants to touch him with a ten inch pole, while Ranieri has coached many good teams with decent results for almost 2 decades.
The only reason Vialli is even thought of as a manager, is because he was a stop gap manager in Chelsea when no one wanted the job, the only reason he was chosen such was because he played there. Heck when he was coach, he was a playing coach, played in his own games etc. It was a lucky chance he got coaching there to begin with, and he has been a total nobody as a coach since his 15 minutes of fame was over in the circus of Chelsea in those days.
Dont be dense and be able to differentiate between Vialli the player and Vialli the coach. Its ridicolous to be blinded by the former. Do you know of great juve players who tried at coaching after retiring and failed? Platini and Zoff, both coached us after retiring, both were bad. Get the significance? Great player is not = to good manager.