The single vehicle is the understanding that all teachings are personal advice on how to soften rigid mind-habits, letting go of pointless aims, and then settling into a gentle presence that is both calm and kind. Our attention simply remains for a while in this way, at ease in itself with nothing that needs to be held or cast away. This aware steadiness is often called shamatha, being calm, and it is the basis for all higher or deeper states of authentic insight.
You may wonder, is mind nothing? It still shimmers and flashes forth, like haze in the heat of the sun. You may wonder, is it something? It has no color or shape to identify it but is utterly empty and completely awake. That is the nature of your mind.