I would use VB
's WebBrowser control for this (you need to add "Microsoft Internet Controls" to your project for this). Drop one on a form, use the Navigate method to open the page you want, then WebBrowserName.document will give the DOM for that page and you can use standard DHTML syntax, like this:
.formname.fieldname.value = "whatever you want"
Note: make sure the page has loaded completely before trying to update the fields - there's an event for this, DownloadComplete or NavigateComplete - I forget which one...