That company is playing with fire if what you said is true

I have never heard that any employer dare not pay employee's final day plus any unused vacation time when the employee resigns. That would be felony crime. You should inqury about it professionally, first by calling, if that doesn't resolve the issue. You should send a written letter, addressed to HR, with registered letter and returned receipt. If it still doesn't get you a good answer. Make sure you document every detail (when you resigned, the specfics dates, etc). You can then file a complaint with EEOC (eeoc.gov). If they intentionally withhold the pay without extremely convincing reasons, it'd be the end of life for a 100-person small company, and could land you a lot of punitive damage award after it goes through a lawsuit. You don't have to be the plaintiff, the EEOC typically is, as long as you can provide enough evidence.