As I understand it, the vortex is scenery, summoned scenery but scenery nonetheless much like a sylvaneth wyldwood. So would it provide cover if the model is entirely within the terrain?
You'd think the Vortex would hurt a model inside it. (Obviously the wizard / spell caster standing on it or riding it would be safe and protected.) What does its scroll say?
I guess the wizard on top gets the same 1 point bonus that models on a watchtower get (for shooting attacks) he/it/she can't be charged at all. But it drives out all other models, friendly or otherwise. The scroll very clearly states that only the model that summoned it can occupy it. All models friend or foe are shoved aside when it is summoned. No model can charge it (so obviously friendly troops can't shelter in it either...although it really ought to say this.)