Vague contracts are enforceable IF the court could reasonable determine what the vague terms meant (if there is no way to determien what was intended, the court will not and cannot enforce it). In this case, from what your write and the context, the quoted language--"Paid in full. Collection has been canceled and I agree not to take further action regarding this matter". --would most likely and reasonable be determined to mean that the *doctor's office* will not take any more legal or collections efforts in regard to this matter. While that's not the only possible interpretation it does seem the most reasonable one, and should not bar you from looking into possible fraud or illegal billing practices.