On the other, other hand, the fact that Kirk & Company can have a "Vulcan exile" in the first place, that they have to volunteer to return to Earth rather than simply being arrested and/or extradited, and that Sarek is Ambassador to
the Federation (rather than, say, a member of the Galactic Senate
) implies that Vulcan is a sovereign state in this situation, which can harbour fugitives if it so chooses.
That has more to do with shifting creative intent than anything else. It's fair to say that, originally, the Federation was meant by the TOS writers to be more of a "UN in space" kind of deal -- hence Sarek being "the Vulcan Ambassador" (his full title is never given, though -- we don't know if he's Vulcan Ambassador to the Federation or Vulcan Ambassador-at-Large, or even if "the Vulcan Ambassador" is his actual title rather than a nickname) and Ambassadors of Federation Member States getting the say over whether Coridan joins the UFP in "Journey to Babel."
I don't think that Spock would've referred to performing actions "at the behest of the Vulcan Ambassador" in a formal briefing to Starfleet Command if all that was meant by the term was "the ambassador who's Vulcan." If nothing else, the term would've been too vague in that context.
But over time, the creative intent has rather obviously shifted to the idea of the Federation as a state in its own right. Ergo, when treating the Federation within the context of the fictional universe it inhabits, we have to treat it as a state.
I would agree with Timo
that various pieces of evidence point to a structure for the Federation which doesn't fit neatly into a present-day analogy as the most likely explanation, and that you're deliberately ignoring such evidence or glossing it over with your thoughts on "creative intent" and interpretation of "Journey to Babel."
As for "Vulcan exhile" -- what makes you think that the Vulcan government even knew that Kirk and Co. were at Mount Seleya? It's entirely possible that the Vulcan government no more knew they were hiding out there than the State of Montana knew that the Unabomber was hiding out in their state.
The various parties speaking before the Federation Council seemed to have a pretty good idea of what happened in Star Trek III
(complete with footage of the Enterprise
), even if the Klingon Ambassador had the wrong impression of who detonated the Genesis Device, and it would seem strange to me for him to ask for the extradition of someone whose location was still unknown.
I'd be very surprised if they knew that the Klingons destroyed the Grissom
and killed David Marcus; and that Kirk blew up the Enterprise,
killed (most of) that Klingon crew, and stole the Bird-of-Prey; but somehow didn't
know what happened afterwards, when he's talking to Sarek, one of the people (a "celebrity," to use Timo's term, and a representative of that government) who met up with them then.