Right now the vassals produce units too fast for testing purposes, it's good they were able to do that though even if they are too powerful right now, it shows their basic combat AI is working. As for them having their own economy and Territory, that's interesting concepts that I will play round with, maybe have them send tribute in the form of a caravan to your town centre? I think the next step at the moment is having the vassals spawn as different civs with different bonuses, one could build more villagers, one could build siege equipment and so on. Having them spawn as mini unique civs as lopess said would be a very cool idea, and opens up more strategies down the line, it's definitely something I'll look into once I get a basis up for having different civs as vassals.

Joan Francesc Oliveras Pallerols https://www.artstation.com/artwork/lGJ46o

Lopess has many small civilizations in the Endovelico mod. Could two people somehow combine them? I find many civilizations in the original game too powerful to be vassals. Also, I think vassals should have territory, perhaps the same color as the empire but with a certain separation. They should also build houses and structures, and have a small economy. I've seen many vassal attacks that are simply overwhelming, not reflecting their true potential. I've observed vassal forces often finish off opponents in battles, not the empire. Besides that, a resource tribute mechanism would be very interesting. You could set a certain percentage and a tribute schedule, and vassals would periodically pay tribute to the empire that controls them. Those are my ideas. What do you think?

That's a good question... I think they potentially would take teh same amount of dmg BUT your attackign unit would not really take any dmg because the unit is facing the wrong direction.  However, I think we would REALLY need to slow down the "turn" speed or something... and that could throw other things out of walk.   But I do like the strategic/historical aspect about trying to ambush, out flank etc.

Pathing clearance is determined in the UnitMotion component, I believe, with PassabilityClass. There are a couple of standard sizes. The bigger the passability class the more problematic the pathing performance.