Do you really need the order IDs to be properties of the object? It might be easier to just return a serialized Dictionary(Of String, String). You could still look up by order ID and it would be easier to loop over than the props of the Javascript object.
Here's an example of what you'd need to do using the dictionary approach:
Dim OrdersDict as New Dictionary(Of String, String)()
For Each Order as BoVendorOrder In Orders
If Not OrdersDict.ContainsKey(Order.VendorOrderID) Then
OrdersDict.Add(Order.VendorOrderID, Vendor.VendorName + " - " + Order.VendorOrderPoNumber)
End If
Next
' Serialize the dictionary object to JSON
' Using System.Web.Script.Serialization.JavascriptSerializer:
Dim Serializer As New JavaScriptSerializer
If MaxLength Then Serializer.MaxJsonLength = Int32.MaxValue
Dim x as String = Serializer.Serialize(OrdersDict) 'Return or response.write x as needed
'or
'Using JSON.net
Dim x as String = JsonConvert.SerializeObject(OrdersDict) 'Return or response.write x as needed