You could come out saying either "This scam is working, I was abled to scam someone using the method" or "This scam method doesn't work and it's a scam itself".
In order to provide a full vouch, you would have to try/use the method, else you could say "It might work, I don't know, I didn't do it", which is pretty much worthless.
Futher, vouching should (I'm aware mostly it doesn't around here) mean to put up your reputation for something.
If you vouch for something, you should be held responsible to a degree (Yes, this doesn't happen around here often, usually "vouches" are just quick ways to get free stuff).