Mikoto nods, as Vivi fixes up a tray for Regulus. "I can place the rest in storage." Which will give her time to listen. Vivi is very nonthreatening for all of how he looks, and Mikoto had noticed the flaw in Regulus's plans when reading earlier - it would have been much more possible to survive the cave if he had made the house elf drink the potion. She is guessing that Regulus will talk to Vivi much more easily than to either her or Telrim.

And Regulus hasn't even bothered to try to hide his curiosity, especially when the little Mage casts Float on the tray to bring it over to the bed. Wandless, nonvocal magic use just reminds him even further of house elves, even if Vivi looks nothing like one.