On portal placement, is correct, it just needs to cover the opening. When lightmass is gathering lighting from any point in the scene, it will take extra samples in the direction of portals. This makes a huge difference on skylight quality in caves / indoors because otherwise the chance that a sample would find that tiny bright opening is very small, which translates to splotchy artifacts. Portals also affect indirect lighting from other sources but those tend to be pretty good quality due to use of photons so the difference is not noticeable.
It doesn’t matter which way the portal is facing. I’m guessing vray does that as an optimization, then Lightmass would only do extra lighting samples on one side of the portal. I didn’t think that was worth the extra complexity.