I'm in the 2nd Quick knockout tourney: 

How many VACATIONS can a contestant take while competing ...

My opponent has taken 'two' ... (the 'first' app 8 days long and after making three moves he takes a  'second' vacation ... now)

When he comes back .. can I take two vacations? or more ... 

How can there be a 'Quick'  tourney if opponents can take 'long' vacations ... "emergencies  exempted"

Where can I find the rule book on this?

its a feature you get after buying a membership, i think you get something like 2 months of total time for vacation in a year. 

Then the tournament may be over before he returns from vacation.

 correction: it should have been over by the time he returns if he/she was there to play his/her games in order to allow the tournament to continue as it should.