I started playing without vassal states lately, and iam not sure i'll ever go back to them 
The system is pretty much flawed imo, it hugely favors the same mechanisms again and again:
Rush out an army, backstab someone (which isn't really hard considering how bad the AI moves), and use vassals to tech/trade stuff for you, as blocker, and just in general as an easy way to gain power.
In Civ3 or Vanilla i had to work hard if i wanted to conquer the world.
Now it is often enough to just take a city or 2, and whoops you get rewarded with owning the whole AI and everything he has.
It also means super AIs are rare now, they will just cap the weaker ones and not use the cities for themselves. With the vassal often being a sitting duck, as @AZ would say
No iam not happy with this, it should not be this easy to win via simple backstabs.

The system is pretty much flawed imo, it hugely favors the same mechanisms again and again:
Rush out an army, backstab someone (which isn't really hard considering how bad the AI moves), and use vassals to tech/trade stuff for you, as blocker, and just in general as an easy way to gain power.
In Civ3 or Vanilla i had to work hard if i wanted to conquer the world.
Now it is often enough to just take a city or 2, and whoops you get rewarded with owning the whole AI and everything he has.
It also means super AIs are rare now, they will just cap the weaker ones and not use the cities for themselves. With the vassal often being a sitting duck, as @AZ would say

No iam not happy with this, it should not be this easy to win via simple backstabs.