Maybe a future expansion or simple update could instate this idea if people like it, then again, maybe it's been addressed before and rejected.

I think it would be good to have the option to turn a civ into a vassal rather than wipe them out. This would be especially useful with the minor races. Instead of just wiping them out, turn them into a provence of your empire. They benefit from your military protection, economy, everything that the people of your civ enjoy but the trade off is that they are basically servants. They must pay a tribute, the method of payment to be decided by the Lord (the superior civ. hereby known as 'you'), and they are also bound to dispatch any military forces they posess to anywhere that you specify for any reason. You also get access to any techs that they have and those that they research in the future. You would also be able to direct their tech advancements, though it would be really slow. If they have a starport you would be able to commission ships from them, at no cost to you. Sounds really good right, yeah it does but we can't leave it at that. It wouldn't add a lot of strategy to the game. No, keeping a vassel couldn't be a walk in the park. There would have to be a risk of an uprising on the vassel worlds. Certain civs would have a naturally higher chance to revolt than others. The rate of tribute would also have an effect on the chance of an uprising. A high rate of tribute would drastically increase the chance of an uprising while a low rate would have an opposite effect and slightly lower the chance. Another thing that would affect the chance of an uprising would be how much you work the vassel civ. The longer you work them by commissioning vessels, the higher the chance becomes. While extended periods of time of not working reduces the chance. One other thing that would affect the chance of an uprising would be how often you use their military and the difficulty and result of the uses. If you constantly use their military and it is constantly being destroyed, then the chance of an uprising goes up. On the other hand, if you rarely use them and their attacks succeed, with or without your help, the chance goes down. If an uprising were to happen, first off you lose all control of the vassel civ. You can no longer commission ships, you cannot see on the planet's surface, nor do you recieve any tribute. The only way to quell it would be to send in ground troops to stop the uprising. If you don't quell the revolt within three months, the civ becomes free again.

Well, what does everyone think?