Is this a dangerous electrical issue?

while in the bathroom I heard the home vacuum cleaner turn on. The vantiy light surrounding the mirror dimmed and STAYED dim through the duration of the vacuum usage. It was plugged into the hall socket right outside the room and obviously on the same circuit. The moment the device went off the lights went back to full illumination. Is this an issue with the device, the circuit, or nothing to worry about? BTW it is a brand new Dyson so it should not be a bad vacuum I hope.