Right now, vassals spawn as gia structures on the map randomly at the start of a match. they can be captured and then become player controlled, once captured they activate their ai, produce workers and a patrolling army, and then produce a small fighting force to attack your enemies. This does mean you can technicaly control their units, but the basic ai implemented will order them back after, usually within 5 seconds of you giving their units orders.
This is a vassal i captured, and as you can see its harvesting basic resources and its army is patrolling its territory.
Edit: i will probbaly have them produce a patrolling army when gia controlled, its already there but ive deactivated this for now for testing purposes but it adds a fight before claiming the vassal prize.
Ideally once i have more time im looking into having these spawn as seperate players once captured, they can still use this basic ai i have in place but it would stop players from acidently selecitng their units.