Wash yams thoroughly. Place in large pot of boiling water. Be sure yams are covered with water. Bring water to boil again. Then simmer yams for about 40 minutes, until tender.
Drain yams. Cool. Peel yams and slice.
In a large skillet, mix margarine, salt, sugar, vanilla, and ½ cup water. Stir over low heat until sugar dissolves. Bring to boil and cook 3 minutes, uncovered.
Reduce heat. Add yams and pecans to mixture in skillet. Heat uncovered for about 5 minutes.
Serve yams alone or with nuts spooned over them.