I Generally use DeveloperFusion to do small converts like that:
http://www.developerfusion.com/tools/convert/csharp-to-vb/[
^]
It suggests
Public Const CartId As String = "TailSpinSpyWorks_CartID"
And
Public Function GetShoppingCartId() As [String]
If Session(CartId) Is Nothing Then
Session(CartId) = If(System.Web.HttpContext.Current.Request.IsAuthenticated, User.Identity.Name, Guid.NewGuid().ToString())
End If
Return Session(CartId).ToString()
End Function
Which look pretty reasonable.