The zoom keybinds should be the only way to zoom in the game, regardless of where you are. Some vehicles such as the Hunter, Ifrit, AH-9, MH-9, etc. still zoom when the right mouse button is clicked, even when the zoom key is not mapped to the right mouse button. All vehicles that allow zooming should also respond to the zoom key, as I have noticed that some do not (although I can't remember how to replicate this).