I also have the Dell UP2715K and a Vega 64. I spent the entire night trying to get it to work, was not easy, not sure what I did or if I can replicate it. it's got something to do with vegaGraphicsfixup.kext . And might be something else, I don't remember anymore, it was late I was trying a hundred things at the same time trying to make it work. It works, fans also work, USB works. I've attached my clover directory. Replace yours with mine and try it out, it should work. Just add your serials numbers in SMBIOS section where I marked it as REMOVED.
Good luck