It's a broken functionality. The only way is if you make your transporting fleet a bridge between two land provinces and they want to go to the other side. I only read about this possibility, never seen it in action.I have a stranded vassal army that I want to ship home. Sent a fleet to the nearest seaport - 1 province away. Turned on the let ally board the fleet. Nothing happened. How do I get my vassal to board the ships so that I can take them home?