It doesn't make sense that the people who use the Dark Side of the Force would call it the "Dark Side" themselves, as they do in the Star Wars movies. People don't usually call their own preferred system something bad. They would put a better spin on it, and call it something like the Full Potential of the Force, or the Unified System of the Force. And they would call the Light Side something like the Lesser Side of the Force, or the Limited System.
So the whole "If you only knew the power of the Dark Side" bit just doesn't ring true. Vader wouldn't have said that. He would have said, "If you only knew the full system of the Force."