Vito is literally giving a speech, to make a certain impression and to achieve certain ends. He does this at several points in the film. It's part of his mystique; it's part of what makes him a good Don.
You're probably correct that everybody already knows what is entailed in entering into a patron/client relationship with Vito. He still makes the speech, because it manipulates his potential new client emotionally, and because it invests their discussion with a ritualistic quality that makes it more than just a transaction.
Note that the soliloquy starts with the words:
What have I ever done to make you treat me so disrespectfully?
Vito has the say the words aloud, rather than just be subtle, because the entire interaction is designed by Vito to use rhetoric to elevate the new relationship into something more than "Hey Mr. Gangster, I can pay you to hurt some guys for me!" The very attributes of Vito's persona that make him an interesting character to watch in a movie also make him someone who can command loyalty from his soldiers and clients - but maintaining that persona requires him to make certain speeches in certain ways.