to get the PHP style MD5 output you have to use FormsAuthentication.HashPasswordForStoringInConfigFile from the System.Web.Security name space.
So you will need to add the system.web.dll to your references.
Also you have to watch the encoding, to match PHP's i had to use Encoding.UTF8.GetBytes().
Thanks for asking the question, because i never knew there was a difference in the outputs.
I wrote mine in C# but should be able to translate to VB.NET pretty easy, but here is my test code
using System;
using System.Text;
using System.Web.Security;
namespace md5
{
class Program
{
static void Main(string[] args)
{
byte[] hash = Encoding.UTF8.GetBytes(FormsAuthentication.HashPasswordForStoringInConfigFile("test", "MD5"));
Console.WriteLine(Convert.ToBase64String(hash));
Console.ReadKey();
}
}
}