Sorin, seigneur de sang impérieux

If the target of the reflexive triggered ability is an illegal target as that ability tries to resolve, it doesn’t resolve. You won’t gain 3 life.

Multiple instances of deathtouch and/or lifelink on the same creature are redundant.

You don’t choose a target for Sorin’s second ability at the time you activate it. Rather, a second “reflexive” ability triggers when you sacrifice a Vampire this way. You choose a target for this ability as it goes on the stack. Each player may respond to this triggered ability as normal.