The best way to verify login success at this high a level is probably to search for an element in the HTML that only appears when logged in. If that exists assume you're logged in, if it doesn't try to navigate and log in again.
Here is a very simple example that uses Len() to check if an element containing text exists. You can be more sophisticated if you want and do things like verify that the information you're seeing matches what you would see if you were logged in.
You can use the same functions you used above to grab elements and then compare any of their members.
Dim IE
Dim Helem
Set IE = CreateObject("InternetExplorer.Application")
IE.Visible = 1
IE.navigate "http://www.example.com"
Set Helem = IE.document.getElementByID("formUsername")
Helem.Value = "username" ' change this to yours
Set Helem = IE.document.getElementByID("formPassword")
Helem.Value = "password" ' change this to yours
Set Helem = IE.document.Forms(0)
Helem.Submit
Do While (IE.Busy)
WScript.Sleep 10
Loop
Dim someElement
Set someElement = IE.document.getElementByID("someElement")
If Len(someElement.innerText) > 0 Then
MsgBox "logged in"
End If