What Vulcan character does Kirstie Alley play in Star Trek - Wrath of Khan?

Kirstie Alley doesn't play any character in Star Trek II: The Wrath of Khan. She was not part of the cast.

You might be thinking of Saavik, who is played by Kirstie Alley in Star Trek III: The Search for Spock and Star Trek IV: The Voyage Home. However, she is not Vulcan, but rather half-Vulcan and half-Romulan.