Seems the answer is not to have a negative amount in the values - it will still process as $190.00 if the amounts are

$100.00

  $10.00

$100.00

Looks like the gateway actually ignores the amounts  provided the correct total is given, so by changing the code to

If result = True Then
                        Dim vDebit As Double = Row.Cells(1).Text
                        Dim vCredit As Double = Row.Cells(2).Text
                        Dim vInvNo As Integer = Row.Cells(8).Text
                        Dim vInvoiceDescription As String = Row.Cells(6).ToolTip
                        Dim LineAmount As Double = 0
                        Dim LineBalance As Decimal = 0
                        If vCredit > 0 Then
                            Dim CreditString As String = "-" & CType(vCredit, Double)
                            LineAmount = CType(CreditString, String)
                            LineBalance = vCredit
                        Else
                            LineAmount = vDebit
                            LineBalance = vDebit
                        End If
                        vTotal += LineAmount
                        objInf.Add("x_line_item=item" & vItemNo, "<|>Invoice " & vInvNo & "<|>" & vInvoiceDescription & "<|>1<|>" & LineBalance & "<|>N")
                        vItemNo += 1
                    End If

It will process the line item as a positive number but the total will be correct