With greater propulsion you mean acceleration? I don't see what this has to do with relativity. Do you mean that E tends to infinity when v approaches c for an object with mass?
A body whether in vacuum or on earth still has inertia. F=dp/dt still holds even in relativity, although you will need to use relativistic momentum for p. The advantage of vacuum is the lack of resistance, like friction, which makes it easier to accelerate.