without the america's contributions, wwii would have probably been a victory for the axis. america sent tons of funding to britain even before entering the war
It could also be said that the Axis wouldn't have been anywhere close to as strong without America's contributions of weapons, raw materials, technology, and other supplies to the Axis powers.
America Selling to Both Sides
As for Civ 4 Peaceful Vassalization, each AI has a "VassalRefuseAttitudeThreshold" XML value, which ranges from Annoyed to Pleased. That AI has to have a higher relationship with you than the corresponding XML value before a Peaceful Vassalization will be possible. The same is true of their attitude towards an AI that they might Peacefully Vasslize under.
So, essentially, if the weak AI is Friendly towards you or towards another AI, then Peaceful Vassalization will be possible, and such a Peaceful Vasslization will even be possible with a Pleased or Cautious relationship, depending upon the AI's relevant XML value.
Interestingly, when I look at the "VassalRefuseAttitudeThreshold" XML values for the Warlords expansion, the values range from "Cautious" to "Friendly," which to me implies that certain AIs will NEVER Peacefully Vassalize in Warlords, since their XML values are "Friendly" and you can't get above "Friendly" in the game. Said AIs are:
Alexander
Genghis Khan
Julius Caesar
Napoleon
Shaka
I'm not certain if the Peace Vassalization Attitude is a Team or Individual Attitude, but I imagine that it is a Team Attitude, meaning that the average (rounded down) of the weaker AI's Attitudes towards you and each of your Vassals (and teammates and permanent allies) would also be factored in. Thus, it's probably a lot easier to get a Peace Vassal before you have too many Vassals or when each of you and your Vassals share a common Religion with the weaker AI that you are trying to take on as a Peace Vassal. Of course, if the weaker AI has a low Attitude threshhold, then you can probably still have several Vassals and make it work. For example, in BtS, the following AIs only require a Cautious or better Attitude for them to consider becoming someone's Peace Vassal:
Catherine
Frederick
Gandhi
Louis XIV
Mansa Musa
Then there's a "iVassalPowerModifier" XML value which seems to influence how much of an influence a player's (or perhaps a team's?) Power level will have in terms of an effect on the willingness to become a Peace Vassal.
For example, the following AIs have a -20 value in BtS, which presumably means that you can make them into a Peace Vassal with a lower-than-normal Power rating:
Catherine
De Gaulle
Gandhi
Mansa Musa
It should be no surprise to see Mansa Musa and Gandhi on both of the latter two lists, since those two AIs can often be seen becoming Peace Vassals in BtS games.