My two cents-
Evans will not win the tour. He will suffer big time in the next two stages for several reasons. First of all, as others have noted, the highest of the high mountains are not and will never be his best terrain. He will get gapped off on both the Galibier and the Alpe. He may not finish several minutes behind or anything like that, but I'd bet that he'll lose at least 2 minutes in total to both Contador and whichever of the two Schlecks decides to be a team leader over the next two stages. Contador will not lose time to Evans in the ITT if he's in yellow and therefore starting last.
Voeckler has no chance to win the Tour. He's been at his limit on the climbs the last two days (and managed to lose time to Evans/Contador) and the same was true at Plateau de Beille. With the true favorites only now reaching top form and the hardest stages yet to come, he'll lose minutes (~5ish total). Top 10 is very possible and that would still be a great result for him and his team. The one way he could possibly (not a big possibility mind you) win the Tour would be if both Schlecks and Contador have bad days over the next two days. In this scenario, he could maybe hang on the back of their group as Evans is deposited out the back. He now needs to pick up time on Evans before the ITT while giving up little, if any time to Contador.
Time is running out/has already run out for the Schlecks. That magical advantage they gained over Contador in the first week of the tour is evaporating day by day. Andy doesn't seem to be on top form, or at least is doing a damn fine job of hiding it if he is. Frank is more consistent, but he doesn't usually have the kick to stay with Contador or his brother when they're really feeling strong. At this point, Contador could probably pass up both of them on the GC with a strong ITT performance (one that we all know he is certainly capable of producing on a course like this). For either of them to win, they need to do something they've never done perfectly before--attacking in turn. Having two climbers on the same team who are at a level to compete for the TdF podium is a major asset that they've largely failed to capitalize on. We need to see perfect teamwork from them over the next two days. They can work with Contador to drop Evans, Basso et. al, but then there's the small matter of dispatching the reigning TdF champion. They can do this if they are able to attack in succession repeatedly. They've shown hints of this so far in this year's tour, but need to go full gas sooner than they have before.
Sanchez is the easiest to overlook, but I think that his best bet is to look for a stage win and the mountains classification. He won't be able to stay with Schlecks/Contador all the way to the top the next two days, and doesn't have it in the ITT to stay with Contador or put major time into the Schlecks. He's very average in that department. HOWEVER, a podium place is possible for him if his alliance with Contador from the past few days continues.
Stage predictions:
18: Sanchez to win with Contador finishing in the same time, Schlecks 20-30 seconds back, Evans a minute back, Voeckler way, way back (2+ minutes).
19: F. Schleck to win the stage, but no major time differences among him, his brother, Contador, and Sanchez. Evans could be up there as well, but they'll all be exhausted from the previous day.
20: Martin to win the ITT. Cancellara will be tired from working for the Schlecks in the mountains and the course is hard enough to suit Martin rather than Cancellara. Martin having already ridden (and won) the ITT in the Dauphine is also a clear advantage.
As for the GC picture, Evans will be best of the contenders, with Contador hardly losing any time. Sanchez will only lose 45 seconds to a minute on them, while A. Schleck will lose maybe a minute and a half. F. Schleck's looking at more like 2-2.5 mins lost. Voeckler will be already out of the picture, but should hang on for top 10.