I would question the 15 degree angle provided by the maker. Unless the blade is very thin, I would expect a 15 degree bevel to be much wider than your photos seem to depict.
Are you using an AngleCube or just the detent marks on the main bar?
Assuming you haven’t gone any further, my recommendation would be for you to “paint” the existing bevel with a Magic Marker (Sharpie) and while using a fine grit stone, find the angle which wipes the full width of the bevel.
If the Sharpie trick shows the angle to truly be 15 dps, Mikedoh”s suggestion to buy the WE Low Angle Adapter is a good one. I have one and it works fine.
Also, the Pro Pack 2 adds a riser block to raise the vise about an inch or so. I think this lets you get in a little closer to the vise jaws, which are machined with about a 13.5 degree angle to them. Assuming that you’re stoning an edge that’s mounted higher, you then should be able to do less than 15 degrees.
And of course as you yourself said, maybe you should just pick a more reasonable angle.