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The thing that seems to have been glassed over here is that the Vettii brothers were Freedmen themselves.The following is from the MIT source mentioned above:
The very fact that these two brothers were able to rise from the status of slaves to wealthy merchants speaks to the social mobility within their society. It is theorized that the Vettii brothers made their fortune as wine merchants and were then able to essentially purchase the elite status of freeborn aristocrats.
A video of a Yale Course lecture on Roman Architecture discusses this building at about 26 minutes in, and also concludes that these were probably slaves quarters.
Since the Vettii brothers were in fact former slaves themselves, you could conclude that, even though they kept slaves, their attitude towards the treatment of those slaves may have been more humane then that of the typical Roman. So unless you can find other examples documented this situation may have been atypical.