These include a number of specifications that describe ADFS and WAP (Web Application Proxy).
- [MS-ADFSPIP]: Active Directory Federation Services and Proxy Integration Protocol
- [MS-ADFSOAL]: Active Directory Federation Services OAuth Authorization Code Lookup Protocol
- [MS-ADFSPP]: Active Directory Federation Service (AD FS) Proxy Protocol
- [MS-ADFSWAP]: Active Directory Federation Service (AD FS) Web Agent Protocol
There are tons of questions on the forum around "Do I have to use WAP as the proxy. Can I use any reverse proxy or a load balancer or F5 or Netscaler etc. ?"
You can as long as the proxy you want to use implements the standards.
Good luck with getting your vendor to confirm that they do and to demonstrate this fact.
In addition, your vendor may also need to comply with the rules around
[MS-OFBA]: Office Forms Based Authentication Protocol
if e.g. the request is
- from a Microsoft Office application that relies on the Office Forms Based Authentication (OFBA) Protocol
- from non-Microsoft-Office clients accessing services that implement the OFBA protocol [MS-OFBA] that rely on ADFS for authentication