Preventing one AI from vassaling everyone else

One of the issues I've run into in my past 2 games, is that one AI would get powerful and start obtaining vassal states. It so happened that I had Washington on my right and Ghandi + Tokugawa on my left.

Toku declares on Ghandi, I wait a few turns and declare on Toku, preparing to use my tech lead to take both of them out in one fell swoop, but suddenly toku and ghandi declare peace and suddenly India becomes Washington's vassal. At the same time, Washington already had a vassal.

The only thing, it seems, I could do was to declare preemptively on Ghandi and then take the horrible war weariness penalty.

I don't understand the vassal system at all: how do you prevent from one powerful AI making all the other AIs his vassals and forcing you into a corner for the rest of the game?

It seems like only the AI benefits from this: they enter into these massive alliances and start teching together leaving you somewhere in the stone age.

Do it to them before they do it to you! Or turn off Vassal States.

Keep and eye out for all the other civ's diplomatic attitude towards one another (and towards you), as well as your power rating vs. everyone else's (both total score and #soldiers).

If you're not playing on a sub-large map (and a not-pangea map), often times you'll only really need to deal with 2 (or at most 3) stacks of units in the short term, and you'll have enough time to adequately re-position your troops to cover your flanks.