They can still do this with only one quali lap leaving extra tyres for the race.
Correct.
But this would be very risky. RBR could take the risk of using 1 run in Q3 and have Vettel (possibly) ending up in 2nd or 3rd on the grid. Then on race day, if he is not leading on lap 1, his chance of victory is all but gone. Remember, as has been seen over the last 12 months, he NEEDS to be time trialling. The moment he gets stuck behind another car or has to come through the field, he is nowhere near as dominant.
Put him at the front, with nobody holding him up and the guy is as at least as good as any other driver in F1. I've noticed that he is able to manage his car very well, when he is time trialling.