Rand is drawing a parallel between the broken "motor" she found and an electric generator. Also, in a way what had been removed is now restored (although the "motor" doesn't work yet) foreshadowing some other restoration yet to occur.
A vault also keeps private, or keeps a secret, what is within it. This is yet another foreshadowing of her discovery that John Galt has already secretly been working in the Taggart Terminal. As the book goes on to show, Galt really has been Dagny's private motivation all along, even before she knew Galt existed.
Grames got a reaction from dream_weaver in Dagny placed the remnant of the motor into a vault in one of the tunnels; the vault h