Vieri at his best was better than Trez at his best, and probably the best goal scorer of the last decade too. If Vieri had stuck with one club and I don't mean a useless club like Inter, maybe somewhere like Juve or Milan, his career would have never dropped.
Trez is lucky to be playing in a team that will keep on playing him even when he is playing like sh*t. So thats why he remained on op. Had he been sold to another team after playing one bad season, he would be bouncing around just like Vieri was.
So Trez is the more fortunate player who always had great supply from the likes of Zidane, Del Piero and Nedved, while Vieri was a one man force.