vassals and resources?

[domze]

I don't quite understand this.
According to the manual a vassal has to give you any resource you request or cause war if he denies. If I understand correctly, denying a resource is the only way of getting a master-vassal war.
I play Russia and I got Korea to capitulate. They appear to have the only horses on continent, so I request them - the dialog says something like "time for your tribute..blah.. blah... denial means war... blah blah...". He gives in. So far so good, but a (long) while later he pops up with the "we no longer wish this trade" dialog and stops giving me horses. I call him again and want to press him, but when I select his horses I'm getting the "what would you want..." instead of "time for your tribute..." option in the dialog.
What's going on? He's still my vassal, how come he can suddenly tell me to freak off?

[roadcage]

Got the Chineese (not Mao) to capitulate. I have been trading very favorably with them since. For example

They had one Deer

I had lotsa Cows and Pigs and no interest in trading them to any one else

I know it was very generous of me but they have been very well behaved.

And why I replied......

Went to war with Russia (Stalin) and their Vassal Inca

China joins in (good)

I beat on an Inca city which logically belongs to the Chineese due to placement. I strip it down to one lone defending Rifleman and inflict max damage with fighters. China has several grenadiers in position.

Does China clean up? No....

After 3 turns of re-strafing, I give up and take the city hoping to transfer control to my vassal China.

Not an option at time of conquest ??? Why ???

Not possible to gift the city to China later ??? why ???