Right now it isn't possible to control the position of the visural-wicket Dialog. They are always positioned in the centre of the browser window.
You could raise a feature request at the google code site for a future release. That said, it sounds like you want something that isn't modal? sort of like a drop down dialog window?
If that's the case you may be better off rolling your own solution in Javascript as those sort of UI features tend to be a bit use case specific.
cheers
Richard