MVC ViewModel not posting back - asp.net-mvc

Question

I am using Viewmodel, however when I submit the form, I am not getting the values back.
I have a Model Like this...
Public Class ProductSummaryModel
Property productGUID As Integer
Property productName As String
Property productPrice As Double
Property productSku As String
Property categories As String()
Property ShipsWithin As Nullable(Of Integer)
<AllowHtml>
Property productWebDescription As String
Property ImageMainPath As String
End Class
Then I have a ViewModel Like this...
Public Class ProductViewModel
Public ProductSummary As ProductSummaryModel
End Class
Here is my Controller...
Public Function Add() As ActionResult ' Default View To Add Item
Dim prdsummary As ProductViewModel = New ProductViewModel
Return View("Add", prdsummary)
End Function
in my "Add" View I have following...
<tr>
<td>Price: </td>
<td><%=Html.TextBoxFor(Function(f) f.ProductSummary.productPrice)%></td>
</tr>
Problem is, when I submit the FORM, I cant get the POSTED Form Values.
Public Function AddItem(ByVal Model As ProductViewModel) As ActionResult ' Add Item into DB
Dim ajaxMessage As New AjaxFormResponse
Dim prd As ProductSummaryModel = New ProductSummaryModel
Model.ProductSummary = prd
Dim skuID As Integer = 0
msgbox(Model.ProductSummary.categories) ' Empty
End Function
May there is a better way of handling this.
* UPDATE **
I have figured out what the Problem is...When passing Model to ViewModel I had to do this...
Dim Product As ProductViewModel = New ProductViewModel
Dim ProductSummary As New ProductSummaryModel
Product.ProductSummary = ProductSummary
Return View("Add", Product)

Answer 1

Why not use the ProductSummaryModel for binding instead of ProductViewModel. This way you can keep thing simple. Avoid nested bindings if you can.
Also be sure you have using in the view as below, and the HttpPost attr. in the Controller
Product Controller
Public Class ProductController
Inherits Controller
' Main Action
Public Function Main() As ActionResult
Dim prdViewModel As ProductViewModel = New ProductViewModel
prdViewModel.ProductSummary = New ProductSummaryModel
Return View("Main", prdViewModel)
End Function
' Partial Action
Public Function Add() As ActionResult
Dim prdViewModel As ProductViewModel = New ProductViewModel
prdViewModel.ProductSummary = New ProductSummaryModel
Return View("Add", prdViewModel)
End Function
'POST Action from Partial View
<HttpPost()> _
Public Function AddItem(ByVal Model As ProductSummaryModel) As ActionResult ' Add Item into DB
msgbox(Model.productPrice) ' Empty
End Function
End Class
Main View
<%# Page Title="" Language="VB" MasterPageFile="~/Views/Shared/Site.Master"
Inherits="System.Web.Mvc.ViewPage(Of MvcApplication2.ProductViewModel)" %>
<asp:Content ID="manageTitle" ContentPlaceHolderID="TitleContent" runat="server">
Main Page
</asp:Content>
<asp:Content ID="manageContent" ContentPlaceHolderID="MainContent" runat="server">
<%
Html.RenderPartial("_Add", Model.ProductSummary)
%>
</asp:Content>
Partial View
<%# Page Language="VB" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage(Of MvcApplication2.ProductSummaryModel)" %>
<asp:Content ID="registerTitle" ContentPlaceHolderID="TitleContent" runat="server">
Register
</asp:Content>
<asp:Content ID="registerContent" ContentPlaceHolderID="MainContent" runat="server">
<% Using Html.BeginForm("AddItem", "Product")%>
<%: Html.AntiForgeryToken() %>
<tr>
<td>Price: </td>
<td><%: Html.TextBoxFor(Function(f) f.productPrice) %></td>
</tr>
<input type="submit" value="Register" />
<% End Using%>
</asp:Content>
UPDATE VIEW MODELS
Public Class ProductSummaryModel
Property productGUID As Integer
Property productName As String
Property productPrice As Double
Property productSku As String
Property categories As String()
Property ShipsWithin As Nullable(Of Integer)
<AllowHtml>
Property productWebDescription As String
Property ImageMainPath As String
End Class
Public Class ProductViewModel
Public ProductSummary As ProductSummaryModel
End Class

Answer 2

I see two problems :
First, you're overriding posted values when you do this :
Dim prd As ProductSummaryModel = New ProductSummaryModel
Model.ProductSummary = prd
The ProductSummary property in the model coming from your view is lost and you only get default values when you read its properties.
Also, you don't show how you set categories values in your view. Are you allowing user inputs or some other operation for this property ?