Vray LightPortal is quite strange, but it's not good idea to use both Domelight + Portals at same time in Vray because they both will sample the light on top of each other.
In Corona it's quite simple and in lots of situation you can avoid portal completely, environment suffices, but if the window opening gets relatively small you can use them, they will only speed up the noise convergence, they don't alter light (unlike Vray, they're not light sources) or complicate sampling. Just the way you create them is maybe odd.. it's material that you should apply to (ideally 1 poly) plane, orientation shouldn't matter, in front of window openings.