It's not a complete vacuum (hard vacuum) inside the vacuum cleaner, just a lower pressure.
The loudest sound possible in a medium is when the pressure waves alternative between complete vacuum and 2x the pressure of the medium. In standard atmospheric pressure it's about 194dB. Inside a vacuum cleaner it would be slightly less, but you'll never achieve sound that loud, so it doesn't really matter.
TL:DR: No difference in sound levels inside a vacuum cleaner.