Keyboard Layout Creator & Vista
Maybe nobody else will ever have this exact problem, but since I did
manage to fix it, here's the answer if anybody does and then looks
around here as I did a couple of days ago.
The KLC program itself has not changed, but when I tried to install
one layout made under XP & .NET it failed, whereas a different layout
worked. The difference turned out to be that the problem layout
defined the keyboard as pertaining to the "Invariant Language" (IV).
When I switched that to a real language (Basque, EU, which I never
expect to want for itself), the installation went through.