Dim ExistingLayerId As ObjectId
Dim elt As LayerTable = CType(tm.GetObject(db.LayerTableId, OpenMode.ForRead, False), LayerTable)
ExistingLayerId = elt.Item("Your Layer Name")
Dim db As Database = HostApplicationServices.WorkingDatabase
Dim curLay As ObjectId = db.Clayer
db.Clayer = ExistingLayerId
' Do something
db.Clayer = curLay