What's an app vendor here? Or what does that mean in general? I feel like one of the reasons given on other (failed) attempts to switch to libre software has been that other public places / ministries etc still using MS had incompatibilities with e. g. Munich's FOSS stack. Though I think this is something you could overcome if you wanted to, it's not like these problems don't exist (and probably have solutions) elsewhere. I think FOSS software has come pretty far in the last years, I haven't booted my Windows partition in months, so I have a tiny bit of hope that they can make it work.