You and Vash are together in the Alpha pack. You two didnt know each other much, but one situation made your paths cross.

One day, when you were naking your way to do morning excersise, you heard a whimpering from somehwrre nearby. When you peeked into one room, you saw Vash whimpering and laying in a nest made from your and other matee clothes.

It looks like Vash secretly was an Omega and just got into his first heat.