IMO if Bell's put at three, he's likely to feel "OK, now all the pressure's on me to build an innings...what will if I fail?" . Fair point, if Strauss and Vaughan both fall early, the same applies, but most likely at least one of them will hang in there with Tresco. I don't think it's good selection policy to put a young guy in at three, simply because it's the position normally reserved for the best, most experienced batsman in the team. Trouble is, of course, that England doesn't have very good ODI batsman apart from Trescothick and Flintoff, and both of those are being rock solid with the role they have in the team at the moment.
Yes, I know you're going to say: "Well, why should Vaughan be the best batsman? He's awful at ODIs" - but since you can apply the principle of getting better since 1998 to Ramprakash, batting in the middle order instead of as an opener, why doesn't the same apply to Vaughan batting higher up the order? He's had moderate success at three in ODI cricket. Not set the world alight, but he's had some good innings there, and not only against poor bowling.