Eh, I view that as a leftover from the pre-Federation days. Functionally, it pretty much is a Consulate, it just keeps it's old name. It's not impossible the EU states will some day reach full federation and yet still have representatives called ambassadors sent to them (or between them), as a matter of tradition.
A few days ago Joao Vale de Almeida was formally installed as the EU's ambassador to America and has suggested that American officials should regard him as their first point of contact for European discussions and not individual European Ambassadors, he informed Washington DC officials and politicians that under the EU's' new Lisbon Treaty, he will be "leading the show" among European representatives in Washington.
Which seems to have pissed off the British Ambassador for one, not sure about the other 25 European Ambassadors..
In TUC, Spock was referred to as the Federation special envoy, while Sarek was called the Vulcan Ambassador. So there are separate Federation and Member diplomatic entities. In the episode Yesteryear, in a (very slightly) alternate ttimeline, Sarek was the Vulcan ambassador to seventeen different Member planets over the course of thirty years.
The Member planets establish official diplomatic missions to each others planets and with foreign powers too. Meaning the Members are separate political-national states from each other and from the Federation.
The Members have "pooled together" to maintain a interstellar political body called the Federation, to collectively handle interstellar matters for them. Interstellar speed limits would be a interstellar matter, so would interstellar defense.
And yet the Federation was still able to make Vulcan go to war with the Dominion without asking Vulcan's permission, the same way the United States didn't have to ask Ohio's permission to declare war on the Third Reich in the 1940s.
As I understand it, all the Ohio state delegates did voted for war against Germany on December 11,1941.
Of the five declared wars in Americas history, three were delared when Congressinal delegations were selected directly by state governments and in 1941 the Ohio state delegates did represent the Ohio people. So Ohio was asked.
In what episode was Vulcan "made" to go to war? The Vulcan government could have withdrawn from the collective decision the go to war, if necessary by withdrawing from Federation Membership.
The Federation never formally declared war upon the Dominion, I started a thread about this a few months ago.