Sorry, I don't know the answer to that one. Don't know if anyone else can help? Looking at the photos, the Vanderhorn seems to have the same side door shape as the Soplair but it could possibly be a slightly different dimension and of course, the zip size could differ.
Thinking of the way our Clairval annex fits the rest of the awning, I think you'd probably get away with a slight opening size mismatch but the zip is a different matter.
The only thing I can suggest is that you try it and see. You shouldn't have any problems selling the Soplair annex if it's a no go, especially with fibre glass poles.
Randa