Is it illegal for a region to vote to secede from a country?

There are several secession movements going on:

- Catalonia wants to secede from Spain, but Spain will not allow a vote on independence.
- Scotland is planning a vote on independence from the UK which will occur on September 18, 2014.  If the vote is yes, then Scotland will become independent, but it is unclear what that means, like will it have to apply to join the EU and the Commonwealth and can it use the British pound.
- Quebec is still talking about becoming independent of Canada, and may hold another referendum at  some point.
- Venice wants to secede from Italy and is holding a non-binding referendum.

In addition, Kosovo declared its independence from Serbia in 2008.  This differs from the other cases because it was done by their parliament instead of by a vote of the people.  The International Court of Justice issued an advisory opinion  "on 22 July 2010, holding that Kosovo's declaration of independence was not in violation either of general principles of international law, which do not prohibit unilateral declarations of independence".

The Falkland Islands in 2013 held a referendum in which 99% of the population voted to remain a British colony, but Argentina said the vote was illegal.

Is Crimea in the same category as these other movements?  If not, why not?  Kerry says that the vote was illegal.  My question is, what is the grounds for calling it illegal?  Isn't the right of self-determination one of the principles of international law?