That's for fitting a Graflok-type 120 film back. Sliding adaptors do not fit into a Graflok back. The Graflok back itself needs to be removed, and the sliding back goes in its place. And yes, if it fits your VX125, you would need the adapter from Klane as well.
Does the Flexadaptor come with a magnifier?
Although it's marketed by PhaseOne, there shouldn't
be a problem using it with other brand DBs, yes?