How long can passengers survive after ditching in the ocean?
Well, your scenario is a bit unlikely because if the aircraft did ditch in the ocean they would have the emergency rafts and slides to take cover in instead of "floating around in their inflatable vests".

However, if someone were floating in the ocean without any protection, how long they could survive depends on various factors.  Temperature is the biggest factor in the amount of time before they would succumb to hypothermia.  

Take a look at this chart to figure out the average temperature of the area in question.  If we go by the latest news, that is going to be somewhere in the neighborhood of 14C.

This page says that the Expected Time of Survival is 1 to 6 hours.

The same page also says:


  Maximize your chances of surviving by:
  
  
  Wearing a personal flotation device (PFD)
  Adopting a survival position
  Keeping clothing on
  Getting as much of body out of the water as possible
  Remaining still and in place UNLESS a floating object, another person, or the shore is nearby
  Keeping a positive mental outlook (a will to survive really does matter)
  
  
  Swimming isn't recommended unless there is little chance of being
  rescued and shore is less than a mile away. Putting on a PFD after
  falling into cold water is almost impossible – SO WEAR IT. If you find
  yourself in cold water without a PFD and nothing to climb onto, tread
  water. Traditional drown-proofing by repeatedly lowering your head
  into the water and floating speeds up heat loss.


Lots more information is available on that page!