I try to avoid VBScript whenever I can, but I can understand how it is desirable in this situation.

First of all, using a macro to retrieve a sender's email address in Outlook can be a painful and ugly process. The SenderEmailAddress property for an item is only available in Outlook 2003 and later, and thanks to a security patch, chances are you'll see a popup window asking for explicit authorization to share those addresses with the "unknown program" for a few minutes. In this case, you're either stuck with manually clicking the Yes button each time or you'll need to research and install additional software to bypass or automate this popup for you.

Secondly, my personal preference is to retrieve item information from the item's object, not from the form. To do this, you can retrieve the object reference from the Inspector object (the form), and since that Inspector is already open, you can find the Application's ActiveInspector like so:
[VBA]
Set objPostItem = Application.ActiveInspector.CurrentItem
MailNoti.To = objPostItem.SenderEmailAddress
MailNoti.Body = objPostItem.Subject & vbCrLf & Message
[/VBA]

I hope that helps!