'The reason sound can't travel through a vacuum is that sound needs a medium (solid, liquid or gas with real vibrating molecules) and not because it is a longitudinal wave' How does this make sense as there are particles in space which can vibrate. Light waves travel through space hence they reach earth and they also use vibration of particle to transport energy. It seems like these two ideas are contracting themselves. Maybe the thing I don't understand is why longitudinal waves such as sound have to travel through a media.
Electromagnetic waves are produced by oscillating charged particles but they do not need other particles to propagate. Indeed electromagnetic waves are solutions of the Maxwell equations with no sources, i.e. in the vacuum.
On the other hand, mechanical waves need an elastic medium to propagate, regardless of being transverse, longitudinal or mixed waves.
Regarding the particles present in deep space which could propagate sound, I suggest you to read this post.