The popular website vizgin.com is not safe to detect the invisible yahoo messenger users.
Check below image where they are asking to fill yahoo user id and password in order to detect the invisible yahoo messenger users.
Since the Vizgin is not yahoo property and demanding your yahoo email id password, it looks fishy and dangerous to provide the yahoo personal information.
So for time being it is advice able not to use vizgin to detect the Invisible Yahoo Messenger Users.
If you want to detect the hidden or offline yahoo messenger users please check below url: