Who is the vulcan actress she plays a doctor and a leutenant in Star Trek The Next Generation?

The Vulcan actress you are referring to is Roxann Dawson, who played the character B'Elanna Torres in "Star Trek: Voyager." However, the character you might be thinking of from "Star Trek: The Next Generation" is Dr. Beverly Crusher, played by Gates McFadden. If you meant a Vulcan character in TNG, that would be T'Pol, but she was featured in "Star Trek: Enterprise," not TNG.