> but the vendor will argue that nobody else allows characters like that in their name inputs

...and maybe they will even link to this page to support that statement! But, seeing that most of the pages are German, I bet they do accept the usual German "special" letters (ÄÖÜß) in names?

So? Have you considered that the names may need to be eventually processed by people who understand the German alphabet but not all French accents (and certinly won't be able to type Hanzi or arabic or whatever else you expect everyone to support)? Will every system they interact with be able to deal with arbitrary symbols. Does the font of their letterhead support every script?

It's reasonable to expect a German company to deal with German script, less reasonable to expect them to deal with literally every script that someone once thought would be funny to include in Unicode.