How to contact professors for PhD vacancies? I know many master's students who get an opportunity to talk to their potential PhD advisors well before applying. In most cases this opportunity is available by the professors with whom they worked for their master's. Moreover, there are many others who establish a contact with professors by mailing them and enquiring about PhD vacancies. 

My questions are:


How important is knowing an advisor prior to applying for PhD? 
Does an email interaction play a vital role when the department makes its admit decisions?
If email contact is important, can a student send out mails to 2-3 professors (working in the same area) in case one of them does not bother to reply?
To sum up, is applying of any use (especially in top universities) when you have no contacts and only your credentials to bank on?
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.