A lot of good tube amps have lower distortion than a lot of good headphones...so is distortion the issue? I don't believe so. Look at the slew rate, or rise time of tubes and notice that the edges are slightly rounded off compared to solid state...it is what I hear, a natural musical transient event...not so sharp.
Electrons are forced to travel through vacuum and I don't think vacuum conducts better than the electrodes themselves...the electrons are wave signals...I think the vacuum serves as an impedance, or impediment. How about sound waves...water conducts sound waves better than air...air has an impedance to the pressure waves. Of course, I have no evidence...all this exists in my minds eyes.
If you're talking about rounded edges, that'll be more of the soft clipping that tube amps provide. That'll be appreciated by a lead guitarist, not an audiophile. Tubes tend to have more even order harmonic distortion when operating in the linear region and that's most of the tube sound that most people seeking tube smps desire. Another aspect of the traditional tube sound is due to output transformers being nonlinear, saturating and having FR issues. This can be addressed using an OTL design or an expensive quality transformer as is done with good audiophile equipment.
Now to the statement, "Electrons are forced to travel through vacuum." Vacuum is not a conductor, period. It is the potential difference between the plate and cathode that does this. Typically the grid is used to control this flow.