[+1]: Target creature you control gains deathtouch and lifelink until end of turn. If it's a Vampire, put a +1/+1 counter on it.
[+1]: You may sacrifice a Vampire. When you do, Sorin, Imperious Bloodlord deals 3 damage to any target and you gain 3 life.
[−3]: You may put a Vampire creature card from your hand onto the battlefield.
Multiple instances of deathtouch and/or lifelink on the same creature are redundant.
Sorin’s second ability goes on the stack without a target. While that ability resolves, you may sacrifice a Vampire. When you do, the reflexive triggered ability triggers and you pick a target to be dealt damage. This is different from effects that say “If you do . . .” in that players may take actions after you’ve sacrificed the creature but before damage is dealt.
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.