The Force is simply a white light. The Force user or nexus is simply a lens that reflects, refracts and filters that light and we see light and dark side in the colours and patterns that result from the lens. The light is always the same.

With Vitiate's known ability to transfer his consciousness to new hosts, the possibility exists that Sidious is simple a newer incarnation of Vitiate, thousands of years later, having found a suitable force attuned host with enough training to be useful but not quite enough to resist him, at some point Vitiate inhabited Sidious and once again worked his way to becoming The Emperor... Discuss.