I've been using this in an XP environment without any problems to get the Open With dialog:
Dim sei As New SHELLEXECUTEINFO
Dim strFileName As String

sei.cbSize = Len(sei)
sei.hwnd = Me.Handle.ToInt32
sei.lpVerb = "openas"
sei.lpFile = strFileName
sei.nShow = 2

ShellExecuteEx(sei)

But, when I try it in Vista Business with any file extension I get this:
This file does not have a program associated with it for performing this action. Create an association in the Set Associations control panel.

I went to that control panel, and gifs, for instance, definitely have a program associated.

Has anyone encountered this problem? Does anyone know how to fix it?

Thanks