Vesper doesn't really advance the plot, but the group makes it to the desert on lythan. The mustang starts sputtering and a needle on the dash moves to the E indicator. Vesper understands what this means and starts feeling quite a bit worried. They were about to be stranded in a desert with no fuel.
Which is actually kind of funny because the gatewatch just arrived in Amonkhet and they are stranded in a desert. But they don't have a vehicle.