I agree with you here, and I'd also add that Schweinsteiger is far better in a number of areas -- he can create from deep, whereas Vieira could do that, but always with powerful runs which transformed defense into attack. Schweinsteiger was able to run games with his passing. Very simple one touch passing, but he set the tone of the game from a deep position -- and you're right, he is more flexible and tactically astute.
The point I'm making here, though, is that if you place Vieira in his favored CM role, the role in which he was one of the best of his generation, then he will dominate most matches -- especially if he has a beast like Keane playing next to him. I just see that partnership as being above and beyond anything Ince and Schweinsteiger can replicate.
So, if you mean that Schweinsteiger is the better player overall because of his versatility, then I can genuinely understand your point. I might even be tempted to agree with you. But, I just can't fathom blanket statements implying that Schweinsteiger was a level above Vieira in every aspect (i.e., put Schweinsteiger in the Vieira role and he'll do just as well -- he wouldn't, imo). However, Vieira isn't capable of playing a number of positions that Schweinsteiger played to a high level. So it's an argument with more than one set of criteria to it.
Maybe I did misunderstand your initial point, though.