I don't see your screenshot or dimensions, but the idea of spanning two vises doesn't sound like it would work too well. Usually, a fixture that locates the workpiece in a location that's repeatable is what's called for in a situation like this. Clear the vises off first, and take some material the surface of the fixture when it's in place, so you're sure it's perfectly flat. Then make a couple of ledgers so stop bars can be aligned to the machine's X and Y axes.