The stone does not accept a vow kept under duress. It says so, in the sixth line, very small, and Corvina finds it after ten minutes of reading badly.

CorvinaYou have to want it at the moment you do it. Not afterwards. Not in general. Then. If you hate it while it is happening the vow is void.

MioneThen I am going to need all three of you to be extremely good at noticing, and I am going to need you to stop when I say.

She takes her own dress off, which is the first thing she does all night that is not the sensible thing, and it takes about nine seconds.