I think you answered your own question?
The issue is that it is not supported. You'll likely get no help if you have any issues, even if you think the issue is not OS related.
Sounds like you are up against a wall, install it and test, test, test before you roll it out.
Don't be surprised it a future installer ends up forbidding the upgrade, you're running on borrowed time.
Thanks for your input. I suspect Vista will work with EPDM 2014 but was hoping someone that has upgraded could verify. I don't really have the time to setup a full test environment. I might check with our VAR to see if they have a working 2014 Web server that I can try to connect to.
Good point about support and upgrades. If it does work I guess I'll hope our rare need for client support continues.