Try
My.Computer.Network.DownloadFile("http://www.youtube.com/get_video?video_id=u98lYTUB6PY&t=vjVQa1PpcFPeQnHdJ-PNP88lxlg9pGt9fq1qMf4PjnU=" & "&fmt=18", "C:\YouTubeVideo.mp4")
Catch ex As Exception
MsgBox(ErrorToString)
End Try
ytd.formats[18].url = "http://" + document.location.host + "/get_video?video_id=" + videoId + "&t=" + token + "&fmt=18"
tube API documentation somewhere here.
?