A new deal is available in the game: cede military control. A vassal may give the master military control over his units(that does not include spies, work boats, workers, settlers or scouts/explorers, corp exec, missionaries, great people, great generals - unless attached to a unit as a warlord etc) The deal is negotiable and friendly vassals are always willing to trade it of a tech or two. The deal lasts for 25(50 marathon) turns and may be renegotiated before it expires, but not cumulatively. Colonies start with this deal being active. The master may upgrade the units he controls in the vassals cities, even if the vassal doesn't have the technology(but needs the resource if required) while paying for the vassals units upgrades from his own pocket.