Unfortunately, I only have French references. The actual French law is at [0]. It says that the buyer is on the hook for any depreciation resulting from handling, apart from what is necessary to determine the characteristics and good working condition of the goods. So opening the vacuum box to see that it's not missing a part or something is fine. Vacuuming your house for 2 weeks is probably not. "Determine the characteristics" is ambiguous to me, so it's not clear where the line is.
For example, Zalando, an online clothes retailer, will accept returns for garments that have been "tried on" but not "worn" [1].
LDLC, an online electronics retailer, will accept returns for items in "original condition and packaging". I expect that if I buy a mouse, use it for a week and there are marks on the pads, they won't accept the return. [2]
[0] https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI0000...
[1] https://www.zalando.fr/aide/Retour-and-Remboursement/Conditi...
https://europa.eu/youreurope/citizens/consumers/shopping/gua...
>If you bought a product or a service online or outside of a shop (by telephone, mail order, from a door-to-door salesperson), you also have the right to cancel and return your order within 14 days, for any reason and without a justification.
In my experience, all retailers (have not tried in France, though) follow this law.