Main point of marches to me is the manpower bonuses. When I'm in a war spree, normal vassals manpower goes down very quickly because it lets its armies trapped a bit easily. Normal vassals end up recruiting mercs to make up for manpower, but it can't be as efficient as a march with much more manpower. I personnaly use marches, but I've always kept vassals around for a long while; I would recon that the downsides are quite of a deterent. I'm not sure what could make the whole system better while keeping balance though.
As for making a march, that's in the diplo view of your vassal. Perhaps you don't have art of war but it's needed to make the march?
As for making a march, that's in the diplo view of your vassal. Perhaps you don't have art of war but it's needed to make the march?