My guess if he’s going to show the truth of the world, showing the world that the leaders are demons would make sense.
One thing we know: what vegapunk gonna tell will shock all the world, something that even ordinary people of OP world would understand
I think he will change the way we perceive the op world, for instance he can say that the world was flood in the past by wg and there is a big continent below the ocean where is located the ancient kingdom, a technological and peaceful kingdom and the current world is just a shadow of the wonderful past. Maybe he can also give the reason of why wg destroyed the ancient kingdom.
The reason I think VP and Clover doesnt know about Imu is because Imu fooled even the 20 first kings, something that would only be recorded in Rio Poneglyph