Neither the vendor nor the collection agency actually prosecutes you criminally--only the state (i.e. the prosecutor's office) does that. However, either the vendor or the collection agency can refer the matter  to the police and/or prosecutor if they think there was a crime, such as the deliberate passing of a bad check; if the authorities agree that there may have been a crime, they may choose to investigate and/or prosecute. Since the actual prosecution comes from the authorites, other parties are in essence only witnesses, bringing the matter to the authorities' attention--and anyone with an awareness of a potential crime may do so. (So, for example, a bank could report a bad check, too.)