Micro gravity and a vacuum are two different environments. We can create a vacuum on the surface of earth (full of gravity) or have the space station have micro gravity with no vacuum inside the station itself. The depths of space have both micro gravity and a vacuum.

Mass is certainly relevant in micro gravity. Kinetic energy (like a moving bullet) is bound to mass. More mass, more kinetic energy. So if you were to shoot a heavier bullet at the same speed as your vector, the bullet would have a higher energy. Then equal and opposite forces blah blah blah...