I found a vulnerability in an IRS authorized e-file vendor's app. What's next?

A vulnerability was discovered in an IRS authorized e-file vendor's app that exposed people's personally identifiable information. The finder disclosed the issue to the vendor, but it was ignored despite multiple attempts to report it over several months. The finder also tried to contact developers, owners, or executives on LinkedIn, but was unsuccessful. The individual is now seeking a way to report the vulnerability to the IRS, as current channels seem focused on personal exposure. They are looking for advice or similar experiences from others who may have encountered a similar situation.