As soon a Rival-kun shows up, Vice-President certainly seems to brighten up and forget her wish to be with President on Christmas Eve and is quick to accept a party invitation from him instead of waiting to see if the President would ask her out on a date, just the two of them. And, no Vice President, you didn't make plans for Christmas Eve with President, you made plans with Rival-kun. And Rival-kun certainly seems to be hell-bent on keeping President and Vice President separated at the party, but why not, after all, she's his date. He's the one who asked her out on Christmas Eve, not the President.