I can’t say 100%, since I do not own a K8400 Vertex, but I have switched my K8200’s to use RAMPS 1.4 without any issue. I have even figured out how to connect the SD/LCD controller.
I have details about it in my write-up about converting an 8200 to dual heads. I think you will find that you can follow the RAMPS part almost exactly with your printer.