Why does my stored procedure always think there is a value? - stored-procedures

Question

I have an ASP.NET 2010 app hitting a SQL 2005 db. I am attempting to build a stored proc dynamically, depending on the values selected on a search screen. I have done this many times in previosu versions of .net & sql with no problem. However, now, my IF statement always always acts as though no data was passed in. I have debugged the app and am sure that data is being set. I should note that when I run the procedure directly, with or without data, the correct data is returned.
Here is the rather simple stored proc..
ALTER PROCEDURE get_cases_by_search_criteria
#vin as varchar(30) = null
AS
declare #sqlstr varchar(1000)
set #sqlstr = 'SELECT
[Case].CaseID,
[Case].VIN,
[Case].Make,
[Case].Model,
[Case].VehicleYear,
if #vin is not null and #vin <> ''
set #sqlstr = #sqlstr + ' and ' + ('[Case].VIN = ''' + convert(varchar,#vin) + '''')
exec(#sqlstr)
RETURN
And here is the code that calls the stored proc...
Public Function GetCases(ByVal oSearchCriteria As SearchCriteria) As List(Of BE.Case)
Dim lstCase As New List(Of BE.Case)
Dim oCase As BE.Case
Dim oProviderFactory As New ProviderFactory
Dim oConnection As DbConnection
Dim oReader As System.Data.IDataReader
Dim oFactory As DbProviderFactory
Dim oCmd As DbCommand
Dim param1 As System.Data.Common.DbParameter
Try
'call my class to get an instance of the DBProviderFactory class
oFactory = oProviderFactory.GetFactory
'call another class of mine. pass in the DBProviderFactory class which will create a non-provider-specific connection object
oConnection = oProviderFactory.GetProviderConnection(oFactory)
'non-specific create command
oCmd = oConnection.CreateCommand
'non-specific parameter
If oSearchCriteria.VIN.Length = 0 Then
param1 = oFactory.CreateParameter()
param1.ParameterName = "#vin"
param1.DbType = DbType.String
param1.Value = DBNull.Value
oCmd.Parameters.Add(param1)
Else
param1 = oFactory.CreateParameter()
param1.ParameterName = "#vin"
param1.DbType = DbType.String
param1.Value = oSearchCriteria.VIN
oCmd.Parameters.Add(param1)
End If
oCmd.CommandType = CommandType.StoredProcedure
oCmd.CommandText = "get_cases_by_search_criteria"
Using (oConnection)
oConnection.Open()
oReader = oCmd.ExecuteReader()
While oReader.Read
oCase = New BE.Case
'Case
If oReader("CaseID") IsNot System.DBNull.Value Then oCase.CaseID = oReader("CaseID")
If oReader("Make") IsNot System.DBNull.Value Then oCase.Make = oReader("Make")
If oReader("Model") IsNot System.DBNull.Value Then oCase.Model = oReader("Model")
If oReader("VehicleYear") IsNot System.DBNull.Value Then oCase.VehicleYear = oReader("VehicleYear")
If oReader("VIN") IsNot System.DBNull.Value Then oCase.VIN = oReader("VIN")
lstCase.Add(oCase)
End While
oConnection.Close()
End Using
Catch ex As Exception
Throw ex
End Try
Return lstCase
End Function