Since the functionality is the same for these two products, owners of Perspective Efex should be offered to purchase Viewpoint at a discount, for example 50 %.
I think it’s worth considering that ViewPoint, like all DxO software, is frequently on sale for 30% off or more. And the Nik Collection is a set of eight tools with a fixed price regardless of how many of them one actually uses. I see Perspective Efex as a low-cost bonus for those who find the Nik Collection more useful than PhotoLab - e.g., users of Lightroom.
I think that 30% discount is not enough because Perspective Efex already has the full functional capability of Viewpoint, but not its integration within Photolab.