Hey Pete,
You gots to pay attention. No one is saying not to use the IBS target for informal barnyard, squirrel rifle or vintage matches etc. As a matter of fact, it was already stated that it is good for those purposes. The OP wanted to know why most of the rimfire benchrest matches are ARA or IR50/50 and not IBS. That was answered.
As for vintage matches, go for it. Set up all the vintage matches you want using whatever target you want. I'm sure it's fun, they just won't be sanctioned. I doubt that ARA or IR50/50 will be getting into a specific vintage class for sanctioned matches. If someone wants to shoot a vintage rifle in a sanctioned match, there's nothing stopping them.