This is pretty uncool—this should be a job for the admins. You are guaranteeing that these vendors will have a pain in the ass time getting ahold of the account that they're due. How in god's name are they supposed to find this obscure thread after they realize their handle is registered? Have you tried contacting the aforementioned vendors?
You also say that you "do not have these vendors public keys", which proves you really haven't thought this through at all. This information is available if you spend a minute looking for it. At the bare minimum, you should locate these vendors PGP keys, encrypt the random forum password for their account to them, post it publicly here and attempt to contact them privately however possible through other sites. But really, you should just stop account squatting and these usernames should be released.
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