Getting a vassal should...

[gallego]

...allow you to build that vassal's unique unit and keep their unique building in cities you have conquered from them. The units shouldn't replace all of your existing ones, but you should be able to build them on top of your existing forces. Treat it as a resource: if you have horses in one part of your empire, you can train cavalry anywhere. Likewise, if you take over another civ, you should be able to use their knowledge and methods to train their special troops. Empires throughout history have employed the troops of their member nations in their own armies. For buildings, you should get to keep the unique building in any of the vassal's cities that you captured, but not build new ones in your own cities. These changes would add some flavor and incentives to acquiring vassals.

 

[Gorman Truart]

This seems too easy to exploit.
1)Settle two cities on an island and liberate them.
2)Build large numbers of vassal's UU.
3)Rinse and repeat.
Eventually you can have multiple UU types in your army, giving a huge advantage.