Hello Student , No, the seat allotted for you if you do not join in the respective college your seat will not be remained vacant.

If you do  make intimation to the authority of the respective counselling that the allocated colleges to you  will be remain vacant as your leaving the country for certain dates duty your unavoidable reason then the respective authority will give you a chance train seat in the second round if you  made your presence at the second round of counselling. Information you can click on the careers 360 college predictor website : http://medicine.careers360.com/college -predictor-seat -allocation

Hope this will help.