molesworth
Warlord
Yes exactly [X] [1] [2] [Y]
Unless you mean [X] [1] [2] [Y], lol.
(I blame my English, hehehe.)
(Proximity to the vassal capital city may be another factor as there is another game mechanic/feature in CIV IV whereby distance from capital calculates whether or not a gifted city is "liberated" to the recipient - I suppose because within a certain distance a gifted city is considered to be within their "natural" territory. Some players on advanced levels even use this mechanic/feature to improve relations with neighbouring civs; they build a city (usually on a poor site) within 5 tiles of the neighbours capital and then "liberate" it to them to get what is known as the "liberation bonus" which is a boost to relations with the neighbour so that they are much less likely to be aggressive to you.
Isn't CIV IV a marvellously complex game?!