You might try plugging the vacuumnmmnm into a different outlet. My router speed lowers when I turn on my vacuuununmnu, so extra resistance might be causing an issue.
The stepper motor signal is basically a digital signal, where the number of edges up and down determines the distance traveled. The width of the pulses doesn’t affect the amount traveled, so I would be surprised if it was electrical noise. The noise will have pretty low current as well, which compared to the 100mAs or more, would probably end up being negligible. It would have to be missing or adding extra steps, which I would think would be pretty hard to do. This is coming from my theory understanding of steppers, I don’t have a ton of practical experience with them.