Lo podes hacer usando Shell, te detecta el navegador predeterminado y te abre la página.
o ShellExecute que es un poco más largo:
ShellExecute hwnd, "open", "http://www.tu-sitio.com.ar", vbNullString, vbNullString, SW_Normal
y declaras:
Declare Function ShellExecute Lib "shell32.dll" Alias "ShellExecuteA" (ByVal hwnd As Long, ByVal lpOperation As String, ByVal lpFile As String, ByVal lpParameters As String, ByVal lpDirectory As String, ByVal nShowCmd As Long) As Long
Public Const SW_Normal = 1
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