I used to love to bring other civs into my wars of conquest. They weakened the target, and gave him another front to worry about. But now vassalage makes it very risky. Either your enemy capitulates to your ally, depriving you of their tasty cities, or your ally capitulates to your enemy, instantly declaring war on you and bringing a two-army turnaround against you. Do you guys only fight wars one-on-one, or do you have some other strategy to avoid this?