Well, in the so called vacuum of space, I do not accept them.
We have created vacuums on Earth and the laws hold true. What could possibly alter reality so that these physical laws become invalid once you enter space, which, as a vacuum, is no different than ones we have created on Earth? A vacuum is simply the absence of matter and that by itself does not effect physical laws.
Objects do not need to push off of anything in order to accelerate, they need only to have a force applied to them. This is what is encompassed in the vector equation F=ma. A simple explanation - The fuel is ejected, providing a force in the direction of the rocket. That force, in turn, accelerates the rocket in the same direction as the force.