Vegapunk created artificial devil fruits, though not as good as the original so it was considered a failure, after discovering how it affects a persons lineage factor and replicating that process; maybe the thing that was missing is the wavelength that Chopper discovered and used the rumble ball to distort and the SAD chemical ingredient (though I assume Vegapunk has something similar to SAD?). Since each devil fruit has a different wavelength they would need the original devil fruit or the devil fruit user to find the specific wavelength. After all, the SMILE devil fruit can give powers but they look like jumbled up animal puzzles, which is where the wavelength theory comes to patch up this side effect. My theory regarding the rumble ball is that it temporarily awakens the devil fruit by distorting its wavelength, and occasional use of the rumble ball with proper training may permanently awaken the devil fruit.