What happens when a creature enchanted with Soul Link would deal lethal damage to a player?
Soul Link's ability is a triggered ability. From the comprehensive rules:


  510.2. Second, all combat damage that’s been assigned is dealt simultaneously.
  
  ...
  
  510.3. Third, any abilities that triggered on damage being assigned or dealt go on the stack.


State-based checks occur before the triggered ability is resolved, so Player A loses before gaining life from the Soul Link.