The difference between engine saving and fuel saving, can be one of two (only the teams now what applies to them):
1. They have 9 or so fuel mixes. There’s 3 mixes of lean, 3 mixes of normal, 3 mixes of rich. More fuel = more revs = more engine stress. Say a driver needs to go a bit faster to stay ahead after the pits, but they don’t want him to overstress the engine, they might go for normal mix 3 or rich mix 1: more revs than normal, but still being considerate of the engine duration. Save the engine but still make good use of the fuel you have, and you might go to normal mix 1 or lean mix 3: fewer fuel, fewer revs, but still a notch above real fuel-saving.
In this scenario, Webber was probably on lean mix 1: the lean mix to save the engine, and ‘mix 1′ to save fuel on top of that. Vettel was probably on lean mix 3: the lean mix to save the engine, but ‘mix 3′ because he still had enough fuel, having saved it earlier in the race.
2. They have two (or more) settings for how ‘fierce’ the engine is. Not per se more revs, but other details that can put extra stress on the engine. This is ‘turning the wick down’: you put the engine in a mode that saves the engine more. Now besides this, there are an x amount of fuel mixes to choose from, ranging from lean to rich; fewer revs to more revs.
In this scenario, both Webber and Vettel had ‘turned their wick down’, but as Vettel had saved fuel earlier in the race, he could run more revs within the engine save mode.