How to contact professors for PhD vacancies?
To supplement the other answers, here are some do's and don'ts for email contact. 


  The worst thing you can do is make it seem like you're trying to cast
  a wide net and don't have a clear focus. That's a guaranteed delete.


Therefore, 


Narrow your search to the people whose work you're really interested in
Read their papers (especially recent ones - I've had people email me about stuff I did 5 year ago - I've moved on :))
Think about their work. Find something intelligent to say (even a question).  
Email the professor and focus on those questions. 


That's most likely to get my (their) attention.