I think your method looks correct. However, be a little careful with how you name variables etc so that you dont interfere with keywords used by the platform in other ways.
For example, as a quick test, try this in an externder rule, its doing exactly the same as your example above but with hardcoded Scenario name and different named variables:
Dim varScen As String = "NameOfScenario"
Dim Scenarioid As Integer = BRApi.Finance.Members.GetMemberId(si,dimtype.Scenario.Id,varScen)
Dim MyScenarioTypeid As ScenarioType = BRApi.Finance.Scenario.GetScenarioType(si,Scenarioid)
Throw New Exception("ScenarioType of "& varScen &" is: "& MyScenarioTypeid.Name)