Some level "less" than Puppets for a city (either conquered or otherwise)... you get the resources in the terrain, but
1. you don't pay building maintenance, but don't get any gold output from the city ?not sure about trade routes?
2. you don't get any of the culture for social policies (but it doesn't increase social policy costs)
3. you don't get any unhappiness from the pop, but you don't get any of the science either.
For this we probably don't even need vassal states.
Here's an idea for you.
Tribute States (Think Friendship Declarations, but with different features)
This would be like applying 'Friendship declaration' attitude modifier to the diplomacy. However, unlike friendship treaties, this can work on AI who may not even like you, but have decided to pay tribute perhaps due to size or defeat in war. During the tribute period (30 turns and renewable) an AI will be more willing to go to war with you, accept resource trades with good/fair pricing, enter into RAs.
However, unlike friendship aggreements, Tribute agreements open with gold or gpt payment and grants the Master a small culture or science bonus, perhaps applied to the capital. And there is an automatic open borders agreement.
Lastly, there will be a
Tribute Reputation mechanic where a 'Tribute State' will evaluate his time as a Tribute state. This mechanic will be determined by
- Happiness of the Tribute Civ
- Economic performance
- Population size
- Territorial Integrity
This will be a running average the AI checks every 20-60 turns comparing their performance. This will prevent the players from using tribute states as punching bags, or getting them into wars that get their cities conquered while the master retake cities and keep it for himself.
This reputation mechanic will be used by the AI to determine if they should renew their tribute agreement, and if a Tribute state is treated well, they will tell others, which will increase chances of other Civs acquiescing to being a Tribute State. There may be even a reward system where a Great Person is 'given' to the master Civ as a bonus for creating a strong relationship, but the details will obviously need to be refined and tested for balance.
The nice thing about this is that unlike Vassals in Civ4, which a human will never engage in as it means a loss, a human player can willingly become a tribute state as a viable strategy. As the agreement is renewable and not permanent (it is not capitulation but a recognition one's relative position at a point in time), and this opens up the door to exciting things, diplomatically.