I think you might be solving the wrong problem here. Someone could very easily set up an account and have it vouch for a bunch of spam accounts before you figur

I think you might be solving the wrong problem here. Someone could very easily set up an account and have it vouch for a bunch of spam accounts before you figured anything out. To solve that problem you'd have to limit "vouching" privileges to a few trusted accounts, and then you're not getting much of a benefit, IMO.