Well, maybe there is something to it, that i don't see right now... I'm assuming that you're running inside VaultExplorer right now... Why don't you pass the hwnd to ShowDialog(...)?
If you don't have the handle, you can get it via your current Process:
TestForm test = new TestForm();
test.ShowDialog(
System.Diagnostics.Process.GetCurrentProcess().MainWindowHandle
);
Regards,
Martin