The tern legal usually connotes a crime. There apprats to nothing criminal about a company trying to not lose a current customer when making a hiring decision. The vensdor probably has a confidentiality agreement with yor employer for the medical office to comply with their confidentiality obligation to patients and staff.
The ptodpective employer may be conceerned that you are bound by a no fompete or training reimbursemenr with the medical office.
Perhaps you should address yhour quandry with the vendor. They may be able to make an offer conditional upon you getting a clean release from your employer.
Good luck.