Actually you've posed an interesting challenge. Until now I hadn't paid much attention to how clouds are placed in Vue since I've always treated them as random. But to create a single, well-defined contrail in the sky: hmm...

The contrail effect is mostly created by the Lines noise node that's connected to the Transparency in the function editor.

We don't have much control over how the pattern is repeated because it's a noise node. So try replacing it with an image map consisting of a fuzzy, thin horizontal black line on a white background. Vue repeats the image map across the sky, but you can adjust the UV scale to achieve a single contrail like the above example.